JP2016204734A - High strength hot rolled steel sheet and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength hot rolled steel sheet having high yield strength and tensile strength and excellent in toughness and weldability, and a production method therefor.SOLUTION: There is provided the high strength hot rolled steel sheet that is composed of a predetermined component composition and has a micro structure having tempered martensite as a main structure and has values defined in the (1) to (3) formulae respectively free-Ti:0.0001% or more, Ceq:0.50% or more and less than 0.60%, Pcm:0.25% to 0.30%, and further has (i-1) a yield strength of 960 MPa or more and a tensile strength of 980 MPa or more, (i-2) a V notch Charpy absorbed energy at -40°C of 47 J or more, (i-3) a sheet thickness of 3.5 mm to 10 mm. free-Ti=Ti-3.417×N (1) Ceq=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14 (2) Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B (3).SELECTED DRAWING: None

Description

  The present invention relates to a high-strength steel sheet used for structural members such as construction machinery and industrial machinery and a method for producing the same, and in particular, high-strength hot rolling excellent in high yield strength, tensile strength, toughness and weldability. It is related with a steel plate and its manufacturing method.

  The boom of a crane, which is an example of a construction machine, has become longer and larger with the recent increase in the height of construction objects. Therefore, in order to reduce the weight of the boom itself and increase the lifting and carrying capacity, higher yield strength and tensile strength are required for the steel plate as the material.

  By increasing the strength of the steel plate, the thickness of the member can be reduced and the weight can be reduced, and the lifting and carrying capacity can be increased by applying the high strength steel plate without reducing the thickness. It is also possible to optimize the plate thickness for each member or part to reduce the weight and optimize the lifting and carrying capacity.

  In adopting such a method, mechanical properties required for the steel sheet are firstly mechanical properties such as yield strength, tensile strength, and elongation. As a specific example, in a steel sheet having a tensile strength of 980 MPa, the yield strength is 960 MPa or more, the tensile strength is 980 MPa or more, and the total elongation is 12% or more. Moreover, since the use environment under freezing is also assumed, high toughness under freezing is required.

  Until now, in order to increase the strength, a large amount of Ti, Nb or the like was added to the steel sheet, and the precipitation strengthening of this was utilized. For example, in each of the steel sheets described in Patent Documents 1 to 3, the Ti addition amount is increased in order to utilize precipitation strengthening of Ti, but in order to fully utilize precipitation strengthening, 0.12% or more. Therefore, high temperature heating of the slab of 1250 ° C. or higher is essential for the addition of Ti and the solution of Ti.

  However, a high-strength steel sheet utilizing a large amount of Ti addition and precipitation strengthening may be inferior in toughness at low temperatures, and is not suitable as a structural member. In addition, when high temperature heating of the slab is required, there is a problem in that the manufacturing cost is increased, for example, a heating furnace corresponding to high temperature heating is limited and a special manufacturing opportunity for high temperature heating is required.

  Therefore, as a high-strength steel sheet that utilizes structure strengthening instead of precipitation strengthening, a method of improving toughness by performing a quenching process (RQ process) for reheating and quenching the steel sheet after hot rolling and tempering it. May be adopted. In addition, a direct quenching method that omits quenching after reheating may be applied by rapid cooling (DQ treatment) immediately after rolling.

  For example, the steel plates described in Patent Documents 4 to 6 are examples of RQ-treated steel, and are characterized by combining the component composition and the RQ treatment conditions. Patent documents 7 and 8 are examples of DQ-treated steel. Each is characterized by a combination of the component composition and heating-rolling-cooling-heat treatment conditions, and in many cases, a tempering treatment is performed after the quenching treatment.

  When tempering is applied, in many cases, it is generally applied at a temperature below the Ac1 point. As temperature conditions, Patent Document 5 discloses 570 to 720 ° C. and Patent Document 9 describes 650 ° C. The following is disclosed.

  However, the steel sheets described in Patent Documents 4, 7, and 8 also have a low strength and are not suitable for construction machine applications even in a method that utilizes tissue strengthening. In addition, the steel sheet described in Patent Document 5 usually requires two quenching processes for one quenching process, and there is a problem that the manufacturing cost is high because there are many processes.

  Furthermore, although the steel plate described in Patent Document 6 has sufficient strength, the amount of alloy added is large and the cost is high. Furthermore, the carbon equivalent (Ceq), which is related to the hardness of the heat-affected zone during welding, and the weld cracking susceptibility composition (Pcm), which is an index of the ease of weld cracking, are high, and the weldability during the manufacture of parts There is an inferior problem.

Japanese Patent Laid-Open No. 07-138638 Japanese Patent Laid-Open No. 05-230529 JP 05-271865 A Japanese Patent Laid-Open No. 06-145787 Japanese Patent Laid-Open No. 06-346144 Japanese Patent No. 4174401 JP 10-195532 A JP 2000-319726 A

  The present invention has high yield strength (hereinafter sometimes referred to as “YP”) and tensile strength (hereinafter sometimes referred to as “TS”), and has high toughness and weldability in view of the problems of the prior art. An object of the present invention is to provide a high-strength hot-rolled steel sheet excellent in resistance and a method for producing the same.

  The present inventors diligently studied a method for solving the above problems. As a result, if the component composition, hot rolling conditions and post heat treatment conditions are optimized, YP is 960 MPa or higher, TS is 980 MPa or higher, and V notch Charpy absorbed energy (hereinafter referred to as “absorbed energy”) at −40 ° C. In other words, it was found that a high-strength hot-rolled steel sheet having a weldability of 47 J or higher can be obtained, and the present invention was completed.

  The gist of the present invention is as follows.

[1] The component composition is mass%,
C: more than 0.12%, less than 0.20%,
Si: 0.25% or more, 0.50% or less,
Mn: 0.50% or more and less than 1.20%,
P: 0.0005% or more, 0.020% or less,
S: 0.0005% or more, 0.010% or less,
Al: 0.01% or more, 0.07% or less,
N: 0.0005% or more, 0.0060% or less,
Nb: 0.01% or more, 0.04% or less,
Ti: 0.005% or more, 0.030% or less,
B: 0.0005% or more, 0.002% or less,
Cu: 0.10% or more, 0.40% or less,
Ni: 0.10% or more, 0.40% or less,
Cr: more than 0.60%, 1.20% or less,
Mo: 0.10% or more, 0.40% or less, and
The remainder: iron and inevitable impurities
The microstructure is mainly tempered martensite,
The values defined in the following formulas (1), (2), and (3) are respectively
free-Ti: 0.0001% or more Ceq: 0.50% or more, less than 0.60%, and
Pcm: 0.25% or more and 0.30% or less,
(I-1) The yield strength is 960 MPa or more, the tensile strength is 980 MPa or more,
(I-2) V notch Charpy absorbed energy at −40 ° C. is 47 J or more, and
(I-3) A high-strength hot-rolled steel sheet having a thickness of 3.5 mm or more and 10 mm or less.
free-Ti = Ti-3.417 × N (1)
Ti and N are mass% of each element.
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5
+ Mo / 4 + V / 14 (2)
C, Si, Mn, Ni, Cr, and Mo are the mass% of each element.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20
+ Mo / 15 + V / 10 + 5B (3)
C, Si, Mn, Cu, Ni, Cr, Mo, and B are the mass% of each element.

[2] The component composition is further in mass%,
V: 0.001% or more, 0.10% or less,
Ca: 0.0005% or more, 0.0040% or less,
Mg: 0.0005% or more, 0.0040% or less,
REM: The high-strength hot-rolled steel sheet according to [1] above, containing one or more of 0.0005% or more and 0.0040% or less.

[3] A production method for producing the high-strength hot-rolled steel sheet according to [1] or [2],
(Ii-1) The slab having the component composition described in [1] or [2] is heated to 1160 ° C. or higher and lower than 1240 ° C.,
(Ii-2) Hot rolling is performed such that the finish rolling entry temperature is 950 ° C. or more and less than 1100 ° C., and the finish rolling exit side temperature is 880 ° C. or more and less than 940 ° C.,
(Ii-3) The hot-rolled steel sheet is wound into a coil at 550 ° C. or more and less than 650 ° C., cooled to 200 ° C. or less, and then
(Ii-4) Molded into a flat plate, cut into a predetermined length,
(Ii-5) Reheating to 880 ° C. or higher and lower than 940 ° C. and quenching,
(Ii-6) A method for producing a high-strength hot-rolled steel sheet characterized by tempering at 400 ° C. or higher and lower than 550 ° C.

  According to the present invention, it is possible to provide a high-strength hot-rolled steel sheet having high yield strength and tensile strength and excellent weldability and toughness, and a method for producing the same.

  The present invention will be described below.

The high-strength hot-rolled steel sheet of the present invention (hereinafter sometimes referred to as “the present invention steel sheet”) has a component composition of mass%,
C: more than 0.12%, less than 0.20%,
Si: 0.25% or more, 0.50% or less,
Mn: 0.50% or more and less than 1.20%,
P: 0.0005% or more, 0.020% or less,
S: 0.0005% or more, 0.010% or less,
Al: 0.01% or more, 0.07% or less,
N: 0.0005% or more, 0.0060% or less,
Nb: 0.01% or more, 0.04% or less,
Ti: 0.005% or more, 0.030% or less,
B: 0.0005% or more, 0.002% or less,
Cu: 0.10% or more, 0.40% or less,
Ni: 0.10% or more, 0.40% or less,
Cr: more than 0.60%, 1.20% or less,
Mo: 0.10% or more, 0.40% or less, and
The remainder: iron and inevitable impurities
The microstructure is mainly tempered martensite,
The values defined in the following formulas (1), (2), and (3) are respectively
free-Ti: 0.0001% or more,
Ceq: 0.50% or more, less than 0.60%,
Pcm: 0.25% or more and 0.30% or less,
(I-1) The yield strength is 960 MPa or more and the tensile strength is 980 MPa or more,
(I-2) V-notch Charpy absorbed energy at −40 ° C. is 47 J or more,
(I-3) The plate thickness is 3.5 mm or more and 10 mm or less.
free-Ti = Ti-3.417 × N (1)
Ti and N are mass% of each element.
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5
+ Mo / 4 + V / 14 (2)
C, Si, Mn, Ni, Cr, and Mo are the mass% of each element.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20
+ Mo / 15 + V / 10 + 5B (3)
C, Si, Mn, Cu, Ni, Cr, Mo, and B are the mass% of each element.

Moreover, the component composition of the steel sheet of the present invention is further in mass%,
V: 0.001% or more, 0.10% or less,
Ca: 0.0005% or more, 0.0040% or less,
Mg: 0.0005% or more, 0.0040% or less,
REM: It contains 0.0005% or more and 0.0040% or less of 1 type or 2 types or more.

The method for producing a high-strength hot-rolled steel sheet of the present invention (hereinafter sometimes referred to as “the method of the present invention”) is a production method for producing a steel sheet of the present invention,
(Ii-1) Heat the slab of the composition of the steel sheet of the present invention to 1160 ° C or higher and lower than 1240 ° C,
(Ii-2) Hot rolling is performed such that the finish rolling entry temperature is 950 ° C. or more and less than 1100 ° C., and the finish rolling exit side temperature is 880 ° C. or more and less than 940 ° C.,
(Ii-3) The hot-rolled steel sheet is wound into a coil at 550 ° C. or more and less than 650 ° C., cooled to 200 ° C. or less, and then
(Ii-4) Molded into a flat plate, cut into a predetermined length,
(Ii-5) Reheating to 880 ° C. or higher and lower than 940 ° C. and quenching,
(Ii-6) Tempering at 400 ° C. or higher and lower than 550 ° C.

  First, the reason for limiting the component composition of the steel sheet of the present invention and the reason for limiting the numerical values calculated by the above formulas (1), (2), and (3) will be described. “%” Means “% by mass”.

[Ingredient composition]
C: More than 0.12% and less than 0.20% C is an element that ensures the strength of the steel sheet and is the element that contributes most to the strength (hardness) during quenching. If 0.12% or less, the required strength cannot be obtained, so C is more than 0.12%. Preferably it is 0.13% or more. On the other hand, if it is 0.20% or more, the strength becomes excessive and the ductility is lowered, and the weldability and toughness deteriorate, so C is made less than 0.20%. Preferably it is 0.17% or less.

Si: 0.25% or more and 0.50% or less Si is an element that contributes to the improvement of the welded part shape during arc welding as a deoxidizing material and a strengthening element. If it is less than 0.25%, the effect of addition is not sufficiently exhibited, so Si is made 0.25% or more. Preferably it is 0.28% or more. On the other hand, if it exceeds 0.50%, a large amount of Si scale (defects) is generated on the surface of the steel sheet, the surface properties are lowered, and the toughness is liable to be lowered. Preferably it is 0.45% or less.

Mn: 0.50% or more and less than 1.20% Mn is an element that mainly increases the hardenability and increases the steel sheet strength. If it is less than 0.50%, the effect of addition is not sufficiently exhibited, so Mn is 0.50% or more. Preferably it is 0.70% or more. On the other hand, if 1.20% or more, toughness and weldability deteriorate, Mn is made less than 1.20%. Preferably it is less than 1.10%, more preferably less than 1.0%.

P: 0.0005% or more and 0.020% or less P is an element that contributes to improving the strength of the steel sheet, but is also an element that inhibits ductility and toughness. The smaller the amount, the better. However, if the content is reduced to less than 0.0005%, the manufacturing cost increases significantly. Therefore, P is set to 0.0005% or more. Preferably it is 0.0010% or more. On the other hand, if it exceeds 0.020%, ductility and toughness are remarkably lowered, so P is made 0.020% or less. Preferably it is 0.010% or less.

S: 0.0005% or more and 0.010% or less S is an element that generates MnS and inhibits ductility, weldability, and toughness. The smaller the amount, the better. However, if the content is reduced to less than 0.0005%, the manufacturing cost increases significantly, so S is made 0.0005% or more. Preferably it is 0.0007% or more. On the other hand, if it exceeds 0.010%, ductility, weldability, and toughness are remarkably reduced, so S is made 0.010% or less. Preferably it is 0.004% or less.

Al: 0.01% or more and 0.07% or less Al is an element that functions as a deoxidizing material. If it is less than 0.01%, the effect of addition is not sufficiently exhibited, so Al is made 0.01% or more. Preferably it is 0.02% or more. On the other hand, if it exceeds 0.07%, toughness and weldability deteriorate, so Al is made 0.07% or less. Preferably it is 0.04% or less.

N: 0.0005% or more and 0.0060% or less N is an element that forms BN in steel to reduce solid solution B and inhibits hardenability. The smaller the amount, the better. However, if the content is reduced to less than 0.0005%, the manufacturing cost increases significantly. Therefore, N is set to 0.0005% or more. Preferably it is 0.0010% or more. On the other hand, if it exceeds 0.0060%, the hardenability deteriorates, so N is made 0.0060% or less. Preferably it is 0.0050% or less.

Nb: 0.01% or more and 0.04% or less Nb is an element that suppresses recrystallization and suppresses coarsening of crystal grains. Refinement of crystal grains contributes to improvement in yield strength and toughness. If it is less than 0.01%, the effect of addition is not sufficiently exhibited, so Nb is made 0.01% or more. Preferably it is 0.015% or more. On the other hand, if it exceeds 0.04%, toughness and ductility are lowered, and there is a concern that the entire amount does not form a solution during slab heating, so Nb is made 0.04% or less. Preferably it is 0.035% or less.

Ti: 0.005% or more and 0.030% or less Ti, like Nb, is an element that suppresses recrystallization and suppresses coarsening of crystal grains. Further, it is an element that forms TiN at a high temperature to fix and render N harmless, and suppress precipitation of BN to contribute to securing solid solution B. If it is less than 0.005%, the effect of addition is not sufficiently exhibited, so Ti is made 0.005% or more. Preferably it is 0.010% or more. On the other hand, if it exceeds 0.030%, toughness, weldability, and ductility deteriorate, so Ti is made 0.030% or less. Preferably it is 0.025% or less.

B: 0.0005% or more and 0.002% or less B is an element that remarkably improves hardenability. If it is less than 0.0005%, the effect of addition is not sufficiently exhibited, so B is made 0.0005% or more. Preferably it is 0.0007% or more. On the other hand, if it exceeds 0.002%, the effect of addition is saturated and the toughness decreases, so B is made 0.002% or less. Preferably it is 0.0015% or less.

Cu: 0.10% or more and 0.40% or less Cu is an element that contributes to improvement of strength by enhancing hardenability. If it is less than 0.10%, the effect of addition is not sufficiently exhibited, so Cu is made 0.10% or more. Preferably it is 0.20% or more. On the other hand, if it exceeds 0.40%, the toughness and weldability are reduced, and the concern about hot cracking is increased, so Cu is made 0.40% or less. Preferably it is 0.30% or less.

Ni: 0.10% or more and 0.40% or less Ni is an element that contributes to improvement of toughness and suppression of intergranular cracking due to Cu while enhancing hardenability and improving strength. If it is less than 0.10%, the effect of addition is not sufficiently exhibited, so Ni is made 0.10% or more. Preferably it is 0.20% or more. On the other hand, if it exceeds 0.40%, the price of the steel sheet increases because it is an expensive element, so Ni is made 0.40% or less. Preferably it is 0.35% or less.

Cr: more than 0.60% and not more than 1.20% Cr is an element that contributes to improving the corrosion resistance by increasing the hardenability and contributing to the improvement of the strength, as well as the combined addition with Cu and Ni. It is also an element that acts to delay softening during tempering. If it is 0.60% or less, the effect of addition is not sufficiently exhibited, so the content exceeds 0.60%. Preferably it is 0.80% or more. On the other hand, if it exceeds 1.20%, weldability and toughness deteriorate, so Cr is made 1.20% or less. Preferably it is 1.10% or less.

Mo: 0.10% or more and 0.40% or less Mo is an element that increases the hardenability and contributes to the improvement of the strength and delays the softening during tempering. If it is less than 0.10%, the effect of addition is not sufficiently exhibited, so Mo is made 0.10% or more. Preferably it is 0.15% or more. On the other hand, if it exceeds 0.40%, the price of the steel sheet increases because it is an expensive element, so the content is made 0.40% or less. Preferably it is 0.35% or less.

  The steel sheet of the present invention may contain one or more of V, which contributes to improving the strength of the steel sheet, Ca, Mg, and REM that contribute to the improvement of toughness and the suppression of the decrease in ductility.

V: 0.001% or more and 0.10% or less V is an element that contributes to improving the strength by precipitation strengthening. If it is less than 0.001%, the effect of addition is not sufficiently exhibited, so V is made 0.001% or more. Preferably it is 0.003% or more. On the other hand, if it exceeds 0.10%, weldability and toughness deteriorate, so V is made 0.10% or less. Preferably it is 0.08% or less.

Ca: 0.0005% or more, 0.0040% or less Mg: 0.0005% or more, 0.0040% or less REM: 0.0005% or more, 0.0040% or less Ca, Mg, and REM are non-metallic. It is an element that spheroidizes inclusions and contributes to improvement of toughness and suppression of deterioration of ductility. If any of Ca, Mg, and REM is less than 0.0005%, the effect of addition is not sufficiently exhibited. Therefore, any element is made 0.0005% or more. Preferably, any element is 0.0008% or more.

  On the other hand, if any element exceeds 0.0040%, the coarsening of inclusions and the increase in the number become remarkable and the toughness is lowered. Therefore, any element is made 0.0040% or less. Preferably, any element is 0.0035% or less.

  The balance of the component composition of the steel sheet of the present invention is Fe and inevitable impurities. In addition, the steel plate of the present invention may contain an appropriate amount of Sn, As, etc., which are inevitably mixed from the steel raw material, in addition to the above elements, as long as the properties of the steel plate of the present invention are not impaired.

  Next, the microstructure of the steel sheet of the present invention and the indexes that define the composition characteristics and mechanical characteristics will be described.

[Microstructure]
The microstructure is mainly tempered martensite. That is, the microstructure is a structure including tempered martensite with an area ratio of 50% or more. If the as-quenched martensite is the main structure, there is a concern that the toughness is inferior, and if bainite or ferrite is the main structure, there is a concern that the strength is insufficient. Therefore, the main structure of the microstructure needs to be tempered martensite having excellent strength and toughness.

  In the steel sheet of the present invention, tempered martensite with an area ratio of 50% or more can be secured by satisfying the above component composition and manufacturing conditions described later, yield strength 960 MPa or more, tensile strength 980 MPa or more, and −40 A V-notch Charpy absorbed energy at 47 ° C. or higher can be realized.

  The area ratio of tempered martensite is preferably 60% or more, more preferably 70% or more.

[Composition index]
free-Ti: 0.0001% or more Free-Ti defined by the following formula (1) is an important index for obtaining required mechanical properties (yield strength, tensile strength, absorbed energy), and is 0.0001% It is necessary to secure the above.
free-Ti = Ti-3.417 × N (1)
Ti and N are mass% of each element.

  As described above, Ti is an element that forms TiN at a high temperature to fix and render N harmless, suppress precipitation of BN, and contribute to securing solid solution B. However, if the N amount is larger than the Ti amount, the free-Ti calculated by the above equation (1) becomes 0% or less. In this case, since N is not completely detoxified, the effect of adding B is not sufficiently exhibited, and the hardenability is lowered.

  Therefore, it is necessary not only to individually define the Ti amount and the N amount, but also to define the Ti amount and the N amount in association with each other so that 0.0001% or more free-Ti can be secured. The upper limit of free-Ti is not particularly limited because it is determined from the composition of Ti and N.

Ceq: 0.50% or more and less than 0.60% Ceq defined by the following formula (2) is to achieve the required mechanical properties (yield strength, tensile strength, absorbed energy) and the required toughness and weldability. It is necessary to set the value to “0.50% or more and less than 0.60%”.
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5
+ Mo / 4 + V / 14 (2)
C, Si, Mn, Ni, Cr, and Mo are the mass% of each element.

  Ceq is also called a carbon equivalent and is known as an indicator of weldability. The larger Ceq, the greater the hardening of the heat-affected zone during welding and the easier the occurrence of weld cracks. Therefore, Ceq is less than 0.60%. Preferably it is 0.59% or less.

  On the other hand, the strength of the steel sheet increases as Ceq increases. In the steel sheet of the present invention, the main structure of the microstructure is tempered martensite, and both strength and toughness are achieved. However, tempered martensite has lower strength than as-quenched martensite, so the strength is reduced during tempering. It is necessary to set Ceq in anticipation and ensure the required yield strength and tensile strength. Therefore, Ceq is 0.50% or more. Preferably it is 0.51% or more.

Pcm: 0.25% or more and 0.30% or less Pcm defined by the following formula (3) is the same as Ceq, and in the steel sheet of the present invention, required mechanical properties (yield strength, tensile strength, absorbed energy) and toughness And it is an important index for achieving both weldability and needs to be “0.25% or more and 0.30% or less”.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20
+ Mo / 15 + V / 10 + 5B (3)
C, Si, Mn, Cu, Ni, Cr, Mo, and B are the mass% of each element.

  Pcm is also called a weld crack susceptibility composition and is known as an indicator of weldability. The larger Pcm, the higher the cracking sensitivity of the welded portion and the lower the weldability, so that the preheating conditions according to the plate thickness at the time of welding construction become severe. Therefore, since Pcm is preferably as small as possible, it is set to 0.30% or less. Preferably it is 0.29% or less.

  On the other hand, as Pcm increases, the strength of the steel sheet increases, but the component composition needs to be a component composition that allows for a decrease in strength during tempering. Therefore, Pcm needs to be 0.25% or more. Preferably it is 0.26% or more.

[Characteristic index]
Yield strength (YP): 960 MPa
Tensile strength (TS): 980 MPa
The yield strength is 960 MPa or more, and the tensile strength is 980 MPa or more. In the steel sheet of the present invention, a yield strength of 960 MPa or more and a tensile strength of 980 MPa or more can be realized by satisfying both the above component composition and manufacturing conditions described later. If this strength level can be secured, it will be a steel plate suitable for construction machinery members such as crane booms that are long and large, and it will be lifted and transported by reducing the thickness of the member plate and improving the strength of the member. The capacity can be increased.

  Regarding the yield strength, if there is a yield point in the tensile test, the strength at the upper yield point is taken as the yield strength, and in the case of a round type stress-strain curve where no yield point is seen, 0.2% proof stress is obtained. Yield strength.

V-notch Charpy absorbed energy at −40 ° C .: 47 J or more V-notch Charpy absorbed energy at −40 ° C. is set to 47 J or more. The greater the absorbed energy, the higher the resistance to brittle fracture. In general, the criterion of whether or not brittle fracture occurs is 47J. If the steel member has an absorbed energy of 47 J or more, brittle fracture hardly occurs in the temperature environment in which the steel member is used.

  Since the construction machine is also assumed to be used in a cold region, in the steel sheet of the present invention, the V-notch Charpy absorbed energy at −40 ° C. is set to 47 J or more. A V-notch Charpy absorbed energy of 47 J or more at −40 ° C. can be realized by satisfying both the above component composition and the manufacturing conditions described later.

Plate thickness: 3.5 mm or more and 10 mm or less The plate thickness of the steel sheet of the present invention is 3.5 mm or more and 10 mm or less. If the plate thickness is less than 3.5 mm, it is unsuitable for structural members of construction machines, so the plate thickness is 3.5 mm or more. On the other hand, if the plate thickness exceeds 10 mm, the quenching / tempering effect does not penetrate to the inside, so the plate thickness is set to 10 mm or less.

  Next, the method of the present invention will be described.

The method of the present invention
(Ii-1) Heat the slab of the composition of the steel sheet of the present invention to 1160 ° C or higher and lower than 1240 ° C,
(Ii-2) Hot rolling is performed so that the finish rolling entry temperature is 950 ° C. or more and less than 1100 ° C., and the finish rolling exit temperature is 880 ° C. or more and less than 940 ° C.,
(Ii-3) The hot-rolled steel sheet is wound into a coil at 550 ° C. or more and less than 650 ° C., cooled to 200 ° C. or less, and then
(Ii-4) Molded into a flat plate, cut into a predetermined length,
(Ii-5) Reheating to 880 ° C or higher and lower than 940 ° C and quenching,
(Ii-6) Tempering at 400 ° C. or higher and lower than 550 ° C.

  Hereinafter, process conditions of the method of the present invention will be described.

[Hot rolling and winding conditions]
Slab heating temperature: 1160 ° C. or higher and lower than 1240 ° C. Molten steel having the component composition of the steel sheet of the present invention is cast by a conventional method to obtain a steel slab (slab) to be subjected to hot rolling. This steel slab may be a forged or rolled steel ingot, but is preferably manufactured by continuous casting from the viewpoint of productivity. Moreover, what manufactured the steel piece with the thin slab caster etc. may be used.

  Usually, a steel slab is cooled after casting, heated again, and used for hot rolling. In this case, the heating temperature of the steel slab shall be 1160 degreeC or more and less than 1240 degreeC.

  When the heating temperature is less than 1160 ° C., Ti and Nb are not sufficiently dissolved, and the effect of suppressing recrystallization and suppressing the coarsening of crystal grains is not exhibited. Therefore, the heating temperature is set to 1160 ° C. or higher. Preferably it is 1165 degreeC or more.

  On the other hand, when the steel slab is heated to 1240 ° C. or higher, the crystal grain size becomes coarse, the strength decreases, the amount of scale generation increases, the yield decreases, and the fuel cost required for heating increases. Therefore, the heating temperature is set to less than 1240 ° C. Preferably, it is 1230 degrees C or less.

Finishing rolling entry temperature: 950 ° C. or higher and lower than 1100 ° C. Steel strip (slab) heated to 1160 ° C. or higher and lower than 1240 ° C. is subjected to rough rolling and then subjected to finish rolling. At that time, the finish rolling entry temperature is set to 950 ° C. or higher and lower than 1100 ° C. As with the finish rolling exit temperature described later, the finish rolling entry temperature is lower, the finer the crystal grains of the steel sheet structure, and the more effective the improvement in yield strength and toughness. If this is the case, there is a concern that the finish rolling exit temperature described later will be less than 880 ° C., and therefore the finish rolling entrance temperature is 950 ° C. or higher. Preferably it is 960 degreeC or more.

  In addition, since the temperature of the slab after rough rolling waiting on the entry side of the finish rolling mill decreases, the slab after rough rolling is covered with a cover or heated with IH or the like in order to suppress the temperature decrease. Thus, the finish rolling entry temperature is maintained at 950 ° C. or higher.

  If the finish rolling entry side temperature is 1100 ° C. or higher, the crystal grains of the steel sheet structure are not refined, and the yield strength and toughness are not improved. Therefore, the finish rolling entry temperature is set to less than 1100 ° C. Preferably it is 1080 degrees C or less.

  The steel plate temperature decreases each time it passes through a plurality of stages of finish rolling rolls, but in consideration of the temperature decrease, the slab after rough rolling is heated to a high temperature, or the temperature due to processing heat generated during rolling. Offset by rising.

Finishing rolling exit temperature: 880 ° C. or more and less than 940 ° C. The finishing rolling exit temperature is the same as the finishing rolling entry temperature, and the lower the finish rolling exit temperature, the finer the crystal grains of the steel sheet structure, and the better the yield strength and toughness. However, if the finish rolling exit temperature is less than 880 ° C., the rolling load during finishing rolling may increase and reach the rolling load upper limit of the rolling mill. Since it becomes a problem in rolling, the finish rolling outlet temperature is set to 880 ° C. or higher.

  In addition, if the finish rolling exit temperature is less than 880 ° C., two-phase rolling is performed, and the rolling load may be fluctuated, so the finish rolling exit temperature is set to 880 ° C. or higher.

  On the other hand, if the finish rolling exit temperature is 940 ° C. or higher, the crystal grains become coarse, yield strength and toughness decrease, and the amount of scale increases, resulting in a decrease in surface properties. The temperature is less than 940 ° C. Preferably it is 935 degrees C or less.

Winding temperature: 550 ° C. or more and less than 650 ° C. A steel plate of 880 ° C. or more and less than 940 ° C. rolled by finish rolling is cooled with a run-out table and wound into a coil at 550 ° C. or more and less than 650 ° C. If the coiling temperature is less than 550 ° C, the strength of the steel sheet becomes too high, and in the process of forming into a flat plate by the leveler in the next process, the rolling force becomes excessive and cannot be formed, so the coiling temperature is 550 ° C or higher. And

  On the other hand, when the coiling temperature is 650 ° C. or higher, in addition to the increase in the amount of scale generated, internal oxidation occurs after coiling and the surface properties are lowered, so the coiling temperature is set to less than 650 ° C. Preferably it is 645 degrees C or less.

Cooling temperature after winding: 200 ° C. or less The steel sheet wound in a coil shape is cooled to 200 ° C. or less. When the cooling temperature exceeds 200 ° C., there is a concern that the transformation is not sufficiently completed. Therefore, the steel sheet is cooled to 200 ° C. or less, and the transformation is sufficiently completed. Cool to 200 ° C. or lower by leaving it at room temperature. Preferably it cools to 190 degrees C or less. The cooling rate is not particularly limited.

[Molding after cooling, heat treatment conditions]
A steel plate cooled to 200 ° C. or less is passed through a leveler, formed into a flat plate under normal conditions, and cut into a predetermined length. Next, heat treatment (quenching and tempering) is performed on the cut steel sheet.

Reheating temperature for quenching: 880 ° C., less than 940 ° C. The cut steel sheet is reheated to 880 ° C. or more and less than 940 ° C. to perform quenching. When the reheating temperature is less than 880 ° C., the steel sheet structure does not become a single structure of austenite, so the reheating temperature is set to 880 ° C. or higher.

  On the other hand, if the reheating temperature is 940 ° C. or higher, the austenite crystal grains become coarse and the toughness decreases, so the reheating temperature is set to less than 940 ° C. Preferably it is 935 degrees C or less.

  Quenching is performed immediately after the steel plate temperature reaches 880 ° C. or more and less than 940 ° C. by reheating, or is kept at a temperature of 880 ° C. or more and less than 940 ° C. for a required time as long as productivity is not significantly impaired. After that.

Tempering temperature: 400 degreeC or more and less than 550 degreeC The hardened steel plate is reheated and tempered to 400 degreeC or more and less than 550 degreeC. Generally, when a tempered steel sheet is tempered, the yield strength and toughness improve as the tempering temperature rises. The same applies to the steel sheet of the present invention, and the desired yield strength and toughness can be ensured by tempering at 400 ° C. or higher. Therefore, the tempering temperature is set to 400 ° C. or higher.

  On the other hand, when the tempering temperature is 550 ° C. or higher, the tensile strength is lowered and the toughness is lowered, so the tempering temperature is less than 550 ° C. Preferably it is 545 degrees C or less.

  Tempering is performed immediately after the steel sheet temperature reaches 400 ° C. or higher and lower than 550 ° C. by reheating, or is maintained at 400 ° C. or higher and lower than 550 ° C. for a required time within a range that does not significantly impair productivity. To do later.

  As described above, the steel sheet of the present invention having high yield strength and tensile strength and excellent toughness and weldability can be produced by the method of the present invention. If the steel sheet of the present invention is applied to a structural member of a construction machine, the construction machine can be reduced in weight. For example, if applied to a boom of a large crane, the boom itself can be reduced in weight and lifted. The carrying capacity can be increased.

  Hereinafter, examples of the high-strength hot-rolled steel sheet and the manufacturing method thereof according to the present invention will be given and the present invention will be described more specifically. However, the present invention is not limited to the following examples, and The present invention can be implemented with appropriate modifications within a range that can be adapted to the gist, and these are all included in the technical scope of the present invention.

(Example)
Steel having the component composition shown in Table 1 was melted, and hot rolling and heat treatment were performed under the conditions shown in Table 2. After the heat treatment, the mechanical properties and toughness of the hot rolled steel sheet were measured. Table 3 shows the measurement results.

  Mechanical properties were measured for yield strength and tensile strength, and total elongation. Note that the JIS No. 5 tensile test piece was taken from the direction parallel to the plate width direction with the original thickness as it was, and subjected to a tensile test at room temperature.

  The absorbed energy was measured by conducting a V-notch Charpy impact test at -40 ° C. and measuring the absorbed energy. The test piece was collected so that the crack propagation direction was parallel to the plate width direction. In addition, steel sheets with a thickness of 5 mm and 10 mm are subjected to an impact test with the original thickness, and a steel sheet with a thickness of more than 5 mm and less than 10 mm is subjected to uniform thickness reduction grinding on the front and back surfaces and processed to a thickness of 5 mm, and then subjected to an impact test. Steel sheets with a thickness of 3.5 mm or more and less than 5 mm were subjected to an impact test after the front and back surfaces were uniformly reduced and ground to a thickness of 3.3 mm.

  Also, the measured value of absorbed energy is the same as that with a plate thickness of 10 mm, and the measured value with a plate thickness of 5 mm is doubled to be equivalent to the absorbed energy at a thickness of 10 mm. The measured value of 3.3 mm was tripled and converted to an absorbed energy equivalent at a thickness of 10 mm. The measurement results shown in Table 3 are those having a thickness of 10 mm or converted into absorbed energy at a thickness of 10 mm.

  As shown in Tables 1 to 3, Invention Examples (Production Nos. 1, 3, 6, 8, 10, 12, 14, 16, 18, 20, 22 in which the component composition and production conditions are within the scope of the present invention) 24), the yield strength is 960 MPa or more, the tensile strength is 980 MPa or more, the Charpy absorbed energy at −40 ° C. is 47 J or more, and the microstructure of the microstructure, that is, 50% or more is tempered martensite, Excellent weldability.

  On the other hand, although the component composition is within the scope of the present invention, the production conditions are outside the scope of the present invention (Comparative Examples Nos. 2, 4, 5, 7, 9, 11, 13, 15, 17, 19, In 21, 23, 25), one or more of yield strength, tensile strength and absorbed energy are outside the scope of the present invention. Furthermore, production No. As shown in FIG. 5, some of the surface properties are inferior and are not worthy of service.

  Production No. In Comparative Example 2, the slab heating temperature is below the range of the present invention. In the comparative example of 23, the quenching temperature and the tempering temperature exceed the range of the present invention, and in these comparative examples, the yield strength is insufficient and the absorbed energy is insufficient.

  Production No. In the comparative example of 4, the cooling end temperature of the coil exceeds the range of the present invention. In this case, the toughness is insufficient.

  Production No. In the comparative example of 5, the coiling temperature exceeds the range of the present invention. In this case, the surface properties deteriorate and are not worthy of service.

  Production No. In the comparative example of No. 7, the quenching temperature exceeds the range of the present invention. In the comparative example of No. 9, all of the slab heating temperature, the finish rolling entry temperature, and the finish rolling exit temperature exceed the scope of the present invention. In 17 comparative examples, the finish rolling entry temperature exceeds the range of the present invention, and in these comparative examples, the absorbed energy is insufficient.

  Production No. In 11 comparative examples, the quenching temperature is below the range of the present invention, and the yield strength and the tensile strength are insufficient.

  Production No. In the comparative example No. 13, the tempering temperature is below the range of the present invention, and the production No. In the comparative example of No. 21, the finish rolling outlet temperature exceeds the range of the present invention. In the 25 comparative examples, the tempering temperature exceeds the range of the present invention, and in these comparative examples, the yield strength is insufficient.

  Production No. In 15 comparative examples, the finish rolling entry temperature and finish rolling exit temperature are below the range of the present invention. In this case, rolling is difficult due to an increase in rolling load during finish rolling and fluctuations in rolling load, and stable production cannot be achieved.

  Production No. In 19 comparative examples, the slab heating temperature is below the range of the present invention, the tempering temperature is above the range of the present invention, and all of the yield strength, tensile strength, and absorbed energy are insufficient.

  Production No. The comparative examples of Nos. 26 to 36 are comparative examples using steel whose component composition is outside the range of the present invention (refer to steel No. MW in Table 1). Even if manufacturing conditions satisfy the scope of the present invention, one or more of yield strength, tensile strength, absorbed energy, weldability, and tempered martensite fraction are outside the scope of the present invention.

  As described above, it is clear from the invention examples and comparative examples that according to the present invention, a high-strength hot-rolled steel sheet having high yield strength and tensile strength and excellent in toughness and weldability can be provided.

  According to the present invention, it is possible to provide a high-strength hot-rolled steel sheet having high yield strength and tensile strength and excellent toughness and weldability, and a method for producing the same. If the high-strength hot-rolled steel sheet of the present invention is applied to a structural member of a construction machine, the construction machine can be reduced in weight. For example, if it is applied to the boom of a large crane, the boom itself can be reduced in weight and the lifting and carrying capacity can be increased, and the working efficiency is remarkably improved. In addition, due to excellent weldability, it is possible to reduce member manufacturing costs. Moreover, the high toughness improves the reliability of the construction machine when used in a low temperature environment. Therefore, the present invention has high industrial applicability.

Claims (3)

Ingredient composition is mass%,
C: more than 0.12%, less than 0.20%,
Si: 0.25% or more, 0.50% or less,
Mn: 0.50% or more and less than 1.20%,
P: 0.0005% or more, 0.020% or less,
S: 0.0005% or more, 0.010% or less,
Al: 0.01% or more, 0.07% or less,
N: 0.0005% or more, 0.0060% or less,
Nb: 0.01% or more, 0.04% or less,
Ti: 0.005% or more, 0.030% or less,
B: 0.0005% or more, 0.002% or less,
Cu: 0.10% or more, 0.40% or less,
Ni: 0.10% or more, 0.40% or less,
Cr: more than 0.60%, 1.20% or less,
Mo: 0.10% or more, 0.40% or less, and
The remainder: iron and inevitable impurities
The microstructure is mainly tempered martensite,
The values defined in the following formulas (1), (2), and (3) are respectively
free-Ti: 0.0001% or more,
Ceq: 0.50% or more, less than 0.60%, and
Pcm: 0.25% or more and 0.30% or less,
(I-1) The yield strength is 960 MPa or more, the tensile strength is 980 MPa or more,
(I-2) V notch Charpy absorbed energy at −40 ° C. is 47 J or more, and
(I-3) A high-strength hot-rolled steel sheet having a thickness of 3.5 mm or more and 10 mm or less.
free-Ti = Ti-3.417 × N (1)
Ti and N are mass% of each element.
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5
+ Mo / 4 + V / 14 (2)
C, Si, Mn, Ni, Cr, and Mo are the mass% of each element.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20
+ Mo / 15 + V / 10 + 5B (3)
C, Si, Mn, Cu, Ni, Cr, Mo, and B are the mass% of each element. The component composition is further in mass%,
V: 0.001% or more, 0.10% or less,
Ca: 0.0005% or more, 0.0040% or less,
Mg: 0.0005% or more, 0.0040% or less,
REM: 0.0005% or more and 0.0040% or less of 1 type or 2 types or more are contained, The high strength hot-rolled steel plate of Claim 1 characterized by the above-mentioned. A manufacturing method for manufacturing the high-strength hot-rolled steel sheet according to claim 1 or 2,
(Ii-1) The slab having the component composition according to claim 1 or 2 is heated to 1160 ° C or higher and lower than 1240 ° C,
(Ii-2) Hot rolling is performed such that the finish rolling entry temperature is 950 ° C. or more and less than 1100 ° C., and the finish rolling exit side temperature is 880 ° C. or more and less than 940 ° C.,
(Ii-3) The hot-rolled steel sheet is wound into a coil at 550 ° C. or more and less than 650 ° C., cooled to 200 ° C. or less, and then
(Ii-4) Molded into a flat plate, cut into a predetermined length,
(Ii-5) Reheating to 880 ° C. or higher and lower than 940 ° C. and quenching,
(Ii-6) A method for producing a high-strength hot-rolled steel sheet characterized by tempering at 400 ° C. or higher and lower than 550 ° C.