JP6136547B2 - High yield ratio high strength hot-rolled steel sheet and method for producing the same - Google Patents

Description

  The present invention relates to a high yield ratio high strength hot-rolled steel sheet having a scale layer, which has excellent low temperature toughness and excellent post-coating corrosion resistance in a formed part, and a method for producing the same, and is particularly formed mainly by bending. The present invention relates to a high yield ratio high strength hot rolled steel sheet suitable as a material for booms and frames of construction machinery and a manufacturing method thereof.

  Structural members for construction machinery, for example, booms for cranes for construction machinery, are becoming longer and larger with the recent increase in construction objects. Therefore, in order to reduce the weight of the boom itself and increase the lifting and carrying capacity, the steel plate that is the material of the boom tends to be thinned, and higher yield strength is required.

In the past, in order to increase the strength of steel sheets, a large amount of solid solution strengthening elements such as Si and Mn and precipitation strengthening elements such as Ti and Nb have been added to steel components. For example, Patent Documents 1 to 5 all increase the Ti addition amount in order to utilize Ti precipitation strengthening. Moreover, in patent documents 1-3, in order to fully utilize precipitation strengthening, 0.12% or more of Ti addition and 1250 degreeC or more high temperature heating (high temperature slab heating) with respect to a slab are essential. However, when high-temperature slab heating is performed, it has been a problem that toughness, particularly low-temperature toughness, may decrease.
Patent Documents 6 and 7 are inventions related to high-strength steel sheets that ensure strength and toughness by using martensite or tempered martensite as the main phase, and hot-rolled sheets after hot rolling to (Ms point + 50 ° C) or less. After cooling and then holding at a cooling stop temperature of ± 100 ° C., it is tempered by winding to increase strength and toughness. However, when bending and coating high-strength steel sheets that have been wound at a cooling stop temperature of ± 100 ° C., the coating corrosion resistance of the bent portion (forming portion) may be reduced. It was. Further, even when tempered martensite is used as a base, there is a problem that low temperature toughness may be lowered.

In general, when a steel sheet with a scale layer is coated, the corrosion resistance after the coating process is “(1) Adhesion between scale layer and ground iron” and “(2) Before electrodeposition coating. It is thought that it is greatly influenced by the chemical conversion processability as a process.
As a technique for improving the adhesion between the scale layer and the ground iron, for example, a method in which the structure of the scale layer is mainly magnetite (Fe ₃ O ₄ ) (see, for example, Patent Documents 8 to 10), and thinning is performed. Methods (for example, see Patent Documents 9 to 13) and methods for reducing the ratio of MnFe ₂ O ₄ in the scale layer (for example, see Patent Document 14) are disclosed.
However, in the above-described conventional technology, although the adhesion between the scale layer and the base iron is improved, when the chemical conversion treatment, which is a pretreatment of electrodeposition coating, is performed on the steel sheet with the scale layer, a good chemical conversion treatment film is formed. Therefore, there is a problem that the adhesion with an electrodeposition coating film provided thereafter is lowered and the corrosion resistance after coating is deteriorated. Moreover, when descaling before finish rolling is performed by a high-pressure water descaling device (for example, see Patent Document 15) or the like in order to reduce the scale, chemical conversion processability is not sufficiently obtained, and as a result There is a problem that the adhesion of the electrodeposition coating film is lowered and the coating corrosion resistance is deteriorated.
Patent Document 16 discloses a technique for obtaining excellent coating corrosion resistance by controlling the volume fraction and particle size of the magnetite of the scale. However, in the technique described in Patent Document 16, desired coating corrosion resistance may not be obtained in a portion to which strain is applied by bending.

Japanese Patent Laid-Open No. 07-138638 Japanese Patent Laid-Open No. 05-230529 JP 05-271865 A JP 2002-97545 A JP 2004-250744 A JP 2011-52320 A JP 2011-52321 A JP 09-271806 A JP 2000-87185 A JP 2002-143905 A Japanese Patent Application Laid-Open No. 07-252593 JP 09-272918 A JP 11-277105 A JP 2004-346416 A JP 2000-015323 A JP 2012-20309 A

  The present invention has been made in view of the above problems, and can suppress a decrease in low-temperature toughness, which was a problem of high-strength steel sheets to which Ti and Nb are added, and has excellent post-painting corrosion resistance in a formed portion. An object of the present invention is to provide a high-yield-ratio high-strength hot-rolled steel sheet having a scale layer and a method for producing the same.

First, based on steel containing Ti or Nb that can be expected to be precipitation strengthened, the present inventors have a high yield ratio and bending formability required for a structural member (construction machine member) for a construction machine. The metallographic structure for obtaining low temperature toughness was investigated. As a result, a metal structure with bainite or tempered martensite as the main phase, and suppressing the formation of ferrite and martensite as much as possible, it is possible to achieve both a high yield ratio, good bend formability and excellent low temperature toughness. I found out that there is.
Next, the inventors conducted extensive studies on the relationship between slab heating temperature and low temperature toughness. In general, it is known that in precipitation-strengthened steel utilizing carbides of Ti and Nb, the slab heating temperature is increased to form a solution of Ti and Nb. However, as a result of investigations by the inventors, in the case of a metal structure mainly composed of bainite or tempered martensite, the low temperature toughness deteriorates when the slab heating temperature is high, and rather, the low slab in which Ti and Nb do not form a solution. It has been found that low temperature toughness is greatly improved in the case of heating temperature. The cause is not clear, but when the slab heating temperature is low, the block particle size of bainite and tempered martensite tended to be small, so undissolved Ti and Nb carbonitrides were transformed into bainite or It is speculated that it played some role in the block segmentation of martensitic transformation.

Next, the present inventors investigated in detail the influence of the scale structure on the characteristics of chemical conversion treatment performed as a pretreatment for electrodeposition coating of hot-rolled steel sheets. As a result, the chemical conversion film formed on the surface of the steel sheet by chemical conversion treatment shows a better form as the fraction of magnetite (Fe ₃ O ₄ ) in the scale layer is higher and the magnetite grains are finer. I found out. And since the hot-rolled steel sheet on which a good-form chemical conversion film is formed can improve the adhesion between the electrodeposition coating film formed by electrodeposition coating and the chemical conversion coating film, It was discovered that the corrosion resistance is good.

  Next, the present inventors prepared steel sheets on which scale layers having various scale compositions, scale thicknesses, and magnetite particle sizes were formed in order to clarify the influencing factors of post-coating corrosion resistance of the bend-formed parts. The corrosion resistance after painting after each bending process was evaluated. As a result, the higher the magnetite fraction in the scale layer, the finer the magnetite grains, and the smaller the number of magnetite and iron lamellar eutectoid structures in the scale layer, the more the bend-formed part is coated. It was found that post-corrosion resistance was improved. The cause of this is not clear, but the fracture (cracking) of the scale layer when the scale layer is bent is affected by the particle size of magnetite and the lamellar eutectoid structure of magnetite and iron, causing the scale layer to crack. It is presumed that corrosion has progressed from the part. That is, it can be considered that the progress of corrosion from the fracture portion of the scale layer can be suppressed by making the magnetite grains finer or reducing the lamellar eutectoid structure of magnetite and iron.

Next, the inventors diligently studied the conditions for refining the magnetite grains in the scale layer. As a result, finish rolling is started in the presence of a scale layer having a thickness within a predetermined range, and when an appropriate amount of strain is applied to the scale layer within a predetermined temperature range, it is formed after cooling the steel plate. It has been found that the magnetite crystals become finer. Although the cause of the refinement of the magnetite crystals is not clear, the fineness introduced by the addition of strain in the scale layer mainly composed of wustite (the scale layer formed during high-temperature finish rolling is mainly wustite). It is considered that a small defect may act as a transformation nucleus of magnetite formed during cooling.
Next, the present inventors conducted extensive studies on a method for reducing the ratio of the lamellar eutectoid structure of magnetite and iron. As a result, in finish rolling, an appropriate amount of strain is added to the scale within a predetermined temperature range, and the coiling temperature is set within the appropriate range, so that both the magnetite and iron lamellae formed during winding can be used. It was found that the formation amount of the analysis structure can be reduced.

As a result of each of the above studies, the present inventors have optimized the structure and crystal grain size of the scale layer and the metal structure of the base material by optimizing the components and hot rolling conditions, thereby providing excellent bending workability. It has been found that it is possible to achieve a high yield ratio high strength hot-rolled steel sheet that can further ensure coating corrosion resistance after electrodeposition coating of the bending formed part, and also has good low-temperature toughness. Completed.
The gist of the present invention is as follows.

[1] By mass%
C: 0.05 to 0.15%,
Si: 0.4% or less,
Al: 0.4% or less,
Mn: 1.2 to 2.5%
P: 0.1% or less,
S: 0.01% or less,
N: 0.007% or less,
Ti: 0.03-0.14%,
Nb: 0.008 to 0.06%
B: 0.0003 to 0.0030%
Containing, and
2Mo + Cr: 0.2 to 1.0%
And the balance has a steel component consisting of Fe and inevitable impurities,
Steel structure is bainite and tempered martensite with a total area ratio of 90% or more, martensite and retained austenite with a total area ratio of 5% or less, and ferrite with an area ratio of 10% or less,
And the block particle size of the said bainite and the said tempered martensite is 5 micrometers or less,
Further, the volume fraction of magnetite in the scale layer formed on the surface layer of the steel sheet is 60% or more, the average crystal grain size of the magnetite is 3 μm or less, and the magnetite and iron are co-located in the scale layer. A high-yield-ratio high-strength hot-rolled steel sheet characterized in that the area fraction of the deposited structure is 40% or less, the maximum tensile strength is 780 MPa or more, and the yield ratio is 0.85 or more.
[2] The high yield ratio high strength hot-rolled steel sheet according to the above [1 ], wherein the steel component further contains V: 0.01 to 0.12% by mass.
[3] The above-mentioned [1] , [2], wherein the steel component further contains 0.02 to 2.0% in total of one or two of Cu and Ni by mass% . The high yield ratio high strength hot-rolled steel sheet according to any one of the above.
[4] The above-mentioned [1], wherein the steel component further contains one or more of Ca, Mg, La, and Ce in an amount of 0.0003 to 0.01% in total. ~ High yield ratio high strength hot rolled steel sheet according to any one of the above [3] .

[5] A method for producing the high-strength hot-rolled steel sheet according to any one of [1] to [4] above, wherein slab heating is performed at 1050 to 1200 ° C., and the scale at the start of finish rolling After descaling so that the average thickness of the layer becomes 3 to 30 μm, the steel sheet surface temperature is in the range of 820 to 980 ° C., the cumulative rolling reduction is 40% or more, and the finish rolling finish temperature is 800 ° C. or more. A method for producing a high yield ratio high strength hot-rolled steel sheet, characterized in that finish rolling is performed, and then coiling is performed with a coiling temperature within a range of 200 to 550 ° C.
[6] The high yield ratio high strength hot rolled steel sheet according to the above [5] , wherein after the winding, the coiled hot rolled steel sheet is reheated to a temperature range of 150 to 500 ° C. Method.

According to the high yield ratio high strength hot-rolled steel sheet of the present invention, the above configuration can suppress a decrease in low-temperature toughness, which is a problem of high-strength steel sheets to which Ti or Nb is added, and also has an electrical property in a forming portion. It becomes possible to improve the corrosion resistance after coating. In particular, according to the present invention, even when electrodeposition coating is applied after forming a high-strength hot-rolled steel sheet having a scale layer having the above-described configuration, the adhesion between the scale layer and the ground iron is impaired. Therefore, in addition to excellent low temperature toughness, excellent coating corrosion resistance after molding can be obtained. Thereby, when setting the plate thickness of the conventional high-strength steel plate parts, the plate thickness was set in anticipation of the thickness reduction due to corrosion, whereas according to the high yield ratio high-strength hot-rolled steel plate of the present invention Since excellent coating corrosion resistance can be obtained, it is possible to set the thickness of a part thin, and it is possible to reduce the weight and length of a construction machine to which the part is applied.
Further, according to the method for producing a high yield ratio high strength hot rolled steel sheet of the present invention, a high yield ratio high strength hot rolled steel sheet having excellent coating corrosion resistance and low temperature toughness is produced by adopting the above procedure and conditions. It becomes possible.
In the present invention, “coating” is not limited to electrodeposition baking coating, and other coating may be performed, and the above effect can be obtained in any “coating”.

  Hereinafter, a high yield ratio high strength hot-rolled steel sheet and a method for producing the same according to the present invention will be described in detail. In addition, since this embodiment explains in detail in order to make the meaning of the high yield ratio high strength hot-rolled steel sheet of the present invention and the manufacturing method thereof better understood, the present invention is limited unless otherwise specified. It is not a thing.

[High yield ratio high strength hot-rolled steel sheet]
The high yield ratio high-strength hot-rolled steel sheet of the present embodiment is mass%, C: 0.05 to 0.15%, Si: 0.4% or less, Al: 0.4% or less, Mn: 1.2 -2.5%, P: 0.1% or less, S: 0.01% or less, N: 0.007% or less, Ti: 0.03-0.14%, Nb: 0.008-0.06 %, B: 0.0003 to 0.0030% and 2Mo + Cr: Mo and Cr satisfying 0.2 to 1.0%, with the balance being steel components composed of Fe and inevitable impurities and steel tissue, bainite and tempered martensite in the total area ratio of 90% or more, 5% or less of residual austenite and martensite in the total area ratio, 10% or less of ferrite area ratio, further, the steel sheet The volume fraction of magnetite in the scale layer formed on the surface layer is 60% or more, and The average crystal grain size of the magnetite is 3 μm or less, the area fraction of the eutectoid structure of the magnetite and iron in the scale layer is 40% or less, the maximum tensile strength is 780 MPa or more, and the yield ratio is 0. It is schematically configured to be 85 or more.
In the following, “%” indicating the amount of each element is mass%. Moreover, the remainder of the basic components and selective elements shown below consists of iron and inevitable impurities. Moreover, in the said element, what does not prescribe | regulate a minimum is shown including to an inevitable impurity level.

"Steel component"
Each element which comprises the steel component of the high intensity | strength hot-rolled steel plate of this embodiment is explained in full detail.

“C: Carbon” 0.05 to 0.15%
C is used for controlling the microstructure of steel. If the C content is less than 0.05%, it will be difficult to ensure sufficient tensile strength, so the C content is 0.05% or more. In addition, from the viewpoint of securing tensile strength, the C content is preferably 0.06% or more, and more preferably 0.07% or more. On the other hand, if the amount of C exceeds 0.15%, the low temperature toughness deteriorates. For this reason, the appropriate range of C amount is limited to 0.15% or less. In consideration of the balance between tensile strength and low temperature toughness, the C content is preferably 0.14% or less, and more preferably 0.12% or less.

“Si: silicon” 0.4% or less In the present embodiment, when the Si amount exceeds 0.4%, descaling property is lowered, and as a result, the magnetite fraction in the scale layer is lowered, and coating is performed. Post-corrosion resistance decreases. Furthermore, if the Si content exceeds 0.4%, the ferrite fraction, martensite fraction and retained austenite fraction tend to increase, so the appropriate range is limited to 0.4% or less. From these viewpoints, the Si content is desirably 0.2% or less. The lower limit of the Si amount is not particularly limited, but if it is less than 0.001%, the manufacturing cost increases, so 0.001% is a substantial lower limit.

“Al: Aluminum” 0.4% or less Al is used for deoxidation and metal structure control of a steel sheet. However, if Al exceeds 0.4%, the ferrite fraction increases and the yield ratio decreases. For this reason, the appropriate range is limited to 0.4% or less. The Al content is preferably in a range of 0.2% or less. Moreover, the lower limit of the amount of Al is not particularly limited, but if it is less than 0.001%, the manufacturing cost increases, so 0.001% is a substantial lower limit.

“Mn: Manganese” 1.2-2.5%
Mn is used for securing the strength of steel and controlling the metal structure. If the Mn content is less than 1.2%, the ferrite fraction increases, the low-temperature toughness and bending workability decrease, and it becomes difficult to ensure a sufficient maximum tensile strength. On the other hand, if the amount of Mn exceeds 2.5%, the adhesion between the scale layer and the ground iron is lowered, and the volume fraction of magnetite in the scale layer is lowered. As a result, the corrosion resistance after coating is also lowered. For this reason, the appropriate range of the amount of Mn is limited to the range of 1.2 to 2.5%. From the viewpoint of securing low temperature toughness, bending workability, tensile strength, and post-coating corrosion resistance, the upper limit of the Mn content is preferably 2.3%, and the lower limit is preferably 1.5%.

“P: Phosphorus” 0.1% or less P is used for securing the strength of steel. However, if the P content exceeds 0.1%, the low temperature toughness and weldability deteriorate, so the appropriate range is limited to 0.1% or less. In addition, Preferably, the upper limit of the amount of P is 0.05%. Further, the lower limit of the amount of P is not particularly limited, but if it is less than 0.001%, the production cost in the refining process increases, so 0.001% is a substantial lower limit.

“S: Sulfur” 0.01% or less S is an element that affects the low-temperature toughness of the base material. If the S content exceeds 0.01%, good low temperature toughness cannot be obtained, so the appropriate range is limited to 0.01% or less. Preferably, the upper limit of the amount of S is 0.005%. Further, the lower limit of the amount of S is not particularly limited, but if it is less than 0.0003%, the production cost in the refining process increases, so 0.0003% is a substantial lower limit.

“N: Nitrogen” 0.007% or less When the N content exceeds 0.007%, coarse Ti—Nb-based nitrides are formed, and thus low-temperature toughness decreases. For this reason, the upper limit is limited to 0.007%. In addition, Preferably, the upper limit of N amount is 0.005%. Further, the lower limit of the N amount is not particularly limited, but if it is less than 0.0003%, the production cost increases, so 0.0003% is a substantial lower limit.

"Ti: Titanium" 0.03-0.14%
Ti is used as a precipitation strengthening element and is used for controlling the microstructure and grain size of steel. However, if the Ti content is less than 0.03%, it is difficult to obtain a sufficient tensile strength (TS), and if the Ti content exceeds 0.14%, the low temperature toughness decreases. For this reason, the appropriate range is limited to 0.03 to 0.14%. From the viewpoint of securing tensile strength, the Ti content is preferably 0.05% or more, and from the viewpoint of low temperature toughness, the Ti content is preferably 0.13% or less.

"Nb: Niobium" 0.008-0.06%
Nb is used as a precipitation strengthening element and is used for controlling the metal structure and grain size of steel. However, if the Nb content is less than 0.008%, the low temperature toughness decreases, and if the Nb content exceeds 0.06% , the low temperature toughness and bendability decrease. For this reason, the appropriate range is limited to 0.008 to 0.06% . The Nb content is preferably 0.015% or more from the viewpoint of low temperature toughness, and the Nb content is preferably 0.05% or less from the viewpoint of low temperature toughness and bendability.

“B: Boron” 0.0003 to 0.0030%
B is used for controlling the metal structure of the steel sheet. However, when the amount of B is less than 0.0003%, it becomes difficult to obtain sufficient tensile strength, and low temperature toughness and bendability are deteriorated. On the other hand, if the amount of B exceeds 0.0030%, the low temperature toughness decreases. For this reason, the appropriate range is limited to 0.0003 to 0.0030%. From the viewpoint of securing tensile strength, the B content is preferably 0.0006% or more, and from the viewpoint of low temperature toughness and bendability, the B content is preferably 0.0025% or less.

"Total amount of 2Mo + Cr" 0.2-1.0%
Mo and Cr are used for securing the strength of steel, controlling the metal structure, and improving the post-painting corrosion resistance of the bent portion. If the content of 2Mo + Cr is less than 0.2%, the ferrite fraction increases, the low temperature toughness and bending workability decrease, and it becomes difficult to secure sufficient tensile strength. Further, if the total amount of 2Mo + Cr is less than 0.2%, it is difficult to obtain an effect of improving the corrosion resistance after painting of the bent portion. On the other hand, if the total of 2Mo + Cr exceeds 1.0%, the low temperature toughness deteriorates and the corrosion resistance after coating also decreases. For this reason, the appropriate range of the total amount of 2Mo + Cr is limited to the range of 0.2 to 1.0%. Contact name, from the viewpoint of the coating Sogo improving corrosion resistance, the total amount of 2Mo + Cr is preferably set to 0.8% or less.

“V: Vanadium” 0.01 to 0.12%
In addition to each element constituting the steel component, V may be contained in an amount of 0.01 to 0.12% by mass.
V may be used for adjusting the strength of the steel. However, when the V content is less than 0.01%, the effect is not obtained, and when it exceeds 0.12%, the bendability and the low temperature toughness may be deteriorated. For this reason, it is preferable to make the appropriate range of V amount into 0.01 to 0.12%.

"One or two of Cu and Ni" 0.02 to 2.0% in total amount
Moreover, in addition to each element which comprises the said steel component, you may contain 0.02 to 2.0% of Cu and Ni in total with the mass% further.
Cu and / or Ni may be used for steel structure control. However, if the total content of any one or two of these elements is less than 0.02%, there is no effect as described above with addition, and if it exceeds 2.0%, the corrosion resistance of coating may decrease. There is. For this reason, it is preferable to make the appropriate range of the total amount of these elements 0.02 to 2.0%.

“One or more of Ca, Mg, La, and Ce” in a total amount of 0.0003 to 0.01%
Moreover, in addition to each element which comprises the said steel component, you may contain 0.0003-0.01% of 1 type, or 2 or more types of Ca, Mg, La, and Ce further by the mass%.
Ca, Mg, La, and Ce may be used for deoxidation of steel. However, if the total amount of one or more of these elements is less than 0.0003%, there is no effect, and if it exceeds 0.01%, low temperature toughness and bendability may be reduced. . For this reason, it is preferable that the appropriate range of the total amount of these elements is 0.0003 to 0.01%.

  Note that the steel component in the present embodiment is substantially composed of Fe other than the above-described elements, and the elements other than the above-described elements are not particularly limited and include the inevitable impurities and the effects of the present invention. You may contain the various elements which do not impair an effect suitably.

"Total amount of area ratio of bainite and tempered martensite"
In the metal structure of the hot-rolled steel sheet of this embodiment, the total amount of area ratios of bainite and tempered martensite is a very important factor for ensuring a high yield ratio and excellent low-temperature toughness and bending workability. When the area ratio of bainite and tempered martensite is less than 90%, low temperature toughness and bending workability are inferior, so the appropriate range is 90% or more. In order to further enjoy a high yield ratio and excellent low temperature toughness and bending workability, the total amount of the area ratio of bainite and tempered martensite is more preferably 95% or more. The bainite may be in any form of granular bainite, upper bainite, and lower bainite, but is preferably a lower bainite having a lath-like form.

"Total area ratio of martensite and retained austenite"
The total area ratio of martensite and retained austenite is a very important factor in ensuring a high yield ratio and excellent low temperature toughness and bending workability. When the total amount of the area ratio of martensite and retained austenite exceeds 5%, it becomes difficult to ensure a yield ratio of 0.85 or more of the hot-rolled steel sheet according to this embodiment described later, Since the bending workability is inferior, the appropriate range is made 5% or less. In order to further enjoy a high yield ratio and excellent low temperature toughness and bending workability, the total amount of the area ratio of martensite and retained austenite is more preferably 2% or less.

"Ferrite area ratio"
The area ratio of ferrite is a very important factor in securing a high yield ratio, low temperature toughness and bending workability. If the area ratio of ferrite exceeds 10%, it becomes difficult to ensure a yield ratio of 0.85 or more, and the low temperature toughness and bending workability are inferior, so the appropriate range is made 10% or less. In order to enjoy a high yield ratio and excellent low-temperature toughness and bending workability, the area ratio of ferrite is preferably 5% or less.

"Volume fraction of magnetite in the scale layer"
The volume fraction of magnetite in the scale layer formed on the surface of the hot-rolled steel sheet according to the present embodiment is a very important factor for ensuring the corrosion resistance after electrodeposition coating after forming. That is, the chemical conversion treatment film formed on the surface of the hot-rolled steel sheet by the chemical conversion treatment performed as a pretreatment for electrodeposition coating shows a better form as the magnetite fraction in the scale layer is higher. And since the hot-rolled steel sheet on which a good-form chemical conversion film is formed can improve the adhesion between the electrodeposition coating film formed by electrodeposition coating and the chemical conversion coating film, Corrosion resistance is improved. However, when the magnetite fraction in the scale layer is less than 60%, it becomes difficult to form a good chemical conversion film, and as a result, the adhesion to the coating performed on the chemical conversion film is lowered and the corrosion resistance is deteriorated. For this reason, in this embodiment, the volume fraction of the magnetite in a scale layer is prescribed | regulated to 60% or more. In the present embodiment, it is more preferable that the volume fraction of magnetite in the scale layer is 85% or more from the viewpoint of further improving the corrosion resistance.

"Average grain size of magnetite"
In the hot-rolled steel sheet of the present embodiment, the average crystal grain size of magnetite in the scale layer is a very important factor in securing post-coating corrosion resistance of the formed processed part. If the crystal grain size of the magnetite in the scale layer exceeds 3 μm, it becomes difficult to form a chemical conversion treatment film as a good base, and the corrosion resistance after electrodeposition coating deteriorates. Therefore, the appropriate range of the average crystal grain size of magnetite in the scale layer is 3 μm or less. The average crystal grain size of magnetite in the present embodiment is more preferably in the range of 2 μm or less from the viewpoint of forming a good chemical conversion coating and ensuring post-coating corrosion resistance of the molded portion.
Note that the magnetite described in the present invention, an oxide having a spinel crystal structure consisting of the chemical formula of Fe ₃ O _4. Moreover, in the crystal structure, even when atoms such as Mn, Al, and Ti are partially substituted at the atomic positions of Fe, the effect on the coating corrosion resistance is not changed, but when the substitution rate by other atoms exceeds 30%, the scale Since cracking may be caused, the upper limit is set for the substitution rate of the Fe atom by other atoms.

"Area fraction of eutectoid structure of magnetite and iron"
The area fraction of the lamellar eutectoid structure of magnetite and iron in the scale layer is an extremely important factor for ensuring the post-coating corrosion resistance of the molded part. If the area fraction of the lamellar eutectoid structure of magnetite and iron in the scale layer exceeds 40%, the corrosion resistance after electrodeposition coating deteriorates, so the appropriate range is made 40% or less. In addition, from the viewpoint of forming a good chemical conversion coating film and securing post-coating corrosion resistance of the molded portion, the area fraction of the magnetite-iron lamellar eutectoid structure is preferably 30% or less.

Yield ratio
For example, when applying a hot-rolled steel sheet to a construction machine member, the hot-rolled steel sheet is designed based on the yield strength, from the viewpoint of thinning the hot-rolled steel sheet for the purpose of reducing the weight of the construction machine member. A hot-rolled steel sheet having a high yield strength with respect to the maximum tensile strength is desired. However, if the yield ratio, that is, the value obtained by dividing the yield strength by the maximum tensile strength is less than 0.85, the effects of weight reduction of the construction machine member, lengthening and size increase associated with weight reduction cannot be obtained. Therefore, the yield ratio of the hot-rolled steel sheet in this embodiment is limited to 0.85 or more. In order to further enjoy the thinning effect of the hot-rolled steel sheet, the yield ratio is preferably 0.87 or more.

"Maximum tensile strength"
If the maximum tensile strength of the hot-rolled steel sheet of this embodiment is less than 780 MPa, the effects of reducing the weight, lengthening, and increasing the size of the construction machine member are hardly expected, so the maximum tensile strength is limited to 780 MPa or more.

"Block size of bainite and tempered martensite"
The block particle size of bainite and tempered martensite is an index that correlates with low temperature toughness and bendability. When the block particle diameter exceeds 5 μm, the low temperature toughness and bendability deteriorate, so the appropriate range is limited to 5 μm or less. In addition, the lower limit of the block particle size of bainite and tempered martensite is not particularly limited, but a smaller block particle size is more desirable.
In addition, the block particle size of tempered martensite and bainite was measured by analyzing the 1/4 thickness part of the total thickness of the hot-rolled steel sheet by the EBSD method. Here, the block grain size is an aggregate of laths having the same orientation, and in this embodiment, those having a crystal orientation difference of 5 ° or less between adjacent laths are regarded as the same orientation.

[Method for producing hot-rolled steel sheet]
Next, a method for producing the high yield ratio high strength hot rolled steel sheet of the present embodiment having the steel components and the scale layer having the above configuration will be described.
The manufacturing method of the high yield ratio high strength hot rolled steel sheet according to the present embodiment performs slab heating at 1050 to 1200 ° C., and performs descaling so that the average thickness of the scale layer at the start of finish rolling is 3 to 30 μm. Then, finish rolling is performed with the steel sheet surface temperature in the range of 820 to 980 ° C., the cumulative rolling reduction of 40% or more, and the finish rolling end temperature of 800 ° C. or more, and then the coiling temperature of 200 to 550 ° C. This is a method of winding in a coil shape within the range.
Moreover, in this embodiment, in manufacturing the hot-rolled steel plate having the steel component and the scale layer having the above-described configuration, after the winding is performed in the same procedure and conditions as described above, the temperature is 150 to 500 ° C. It can be set as the method of holding or temperature heating in the range.
Hereinafter, each procedure and each condition prescribed | regulated with the manufacturing method of the hot rolled sheet steel of this embodiment are demonstrated.

First, a slab made of the steel component is heated. By heating the slab in this way, the dissolution and precipitation state of the carbonitride containing Ti and Nb is controlled, and the metal structure and crystal grain size of the steel sheet are controlled, thereby controlling the low temperature toughness of the steel sheet. To do.
When the slab heating temperature is less than 1050 ° C, the low temperature toughness is lowered, and even when it exceeds 1200 ° C, the low temperature toughness is lowered. For this reason, the appropriate condition is limited to 1050-1200 degreeC. From the viewpoint of securing more stable low temperature toughness, the slab heating temperature is more preferably 1170 ° C. or less.

  After heating the slab to the above temperature range, rough rolling and finish rolling are sequentially performed. Under the present circumstances, the conditions of rough rolling are not specifically limited, Each condition conventionally used can be employ | adopted.

In this embodiment, the average thickness (average scale thickness) of the scale layer at the start of finish rolling is an important factor that affects the coating corrosion resistance after hot rolling. Here, in the conventional manufacturing method, usually, descaling is generally performed before finish rolling to remove the scale layer. However, when the average scale thickness at the start of finish rolling is less than 3 μm due to excessive descaling, a fine magnetite crystal cannot be obtained in the scale layer after hot rolling, so a good chemical conversion coating film cannot be obtained. As a result, the corrosion resistance after painting deteriorates. On the other hand, if the average scale thickness at the start of finish rolling exceeds 30 μm, the unevenness of the interface shape between the scale layer and the ground iron after finish rolling becomes large and fatigue characteristics deteriorate, and the magnetite fraction in the scale layer It also causes deterioration of the coating corrosion resistance through a decrease in adhesion and a decrease in adhesion between the scale layer and the steel. For this reason, in the manufacturing method of this embodiment, the appropriate range of the average scale thickness at the start of finish rolling is limited to 3 to 30 μm. In order to form a good chemical conversion film, the average scale thickness is preferably 5 μm or more, and from the viewpoint of securing the magnetite fraction in the scale layer and the adhesion between the scale layer and the ground iron. The average scale thickness is preferably 10 μm or less.
In addition, the descaling method performed before finish rolling is not particularly limited. However, the degree of descaling treatment changes according to the steel composition and the steel plate temperature at the time of descaling, so by changing the water pressure, the amount of water and the injection angle according to these steel components and the steel plate temperature, The average scale thickness after descaling can be adjusted. The average scale thickness can be adjusted by changing the temperature and holding time until the finish rolling is started after the scale is removed by descaling.
Although the scale thickness can be measured by an X-ray diffraction method or cross-sectional structure observation, it is substantially difficult to measure the scale thickness on the spot by these methods in an actual hot-rolled steel sheet production line. For this reason, the temperature and temperature holding conditions after descaling and descaling processing corresponding to the actual machine production line are reproduced in the laboratory, and the scale thickness under these conditions is measured by X-ray diffraction method or cross-sectional observation method (2 It is possible to produce a steel sheet having a desired scale thickness on an actual production line based on the quantitative formula obtained by quantification by means of the average of measurement at a certain place.

Also, in finish rolling, the steel sheet surface temperature and amount of strain applied (cumulative rolling reduction) during rolling are determined by the magnetite crystal grain size in the scale layer after cooling performed after rolling, and the lamellar eutectoid of magnetite and iron. It is an important factor affecting the proportion of tissue.
When the finish rolling is performed under the condition that the surface temperature of the steel sheet is less than 820 ° C., the scale layer is crushed and voids are formed in the scale. As a result, the volume fraction of magnetite in the scale layer is lowered. On the other hand, when finish rolling is performed under conditions where the steel sheet surface temperature exceeds 980 ° C., the magnetite does not become finer after cooling, and the ratio of the eutectoid structure of magnetite and iron increases. For this reason, in the manufacturing method of this embodiment, the appropriate range of the steel plate surface temperature at the time of finish rolling is limited to 820-980 degreeC.

Also, in finish rolling, if the cumulative rolling reduction within the appropriate temperature range of the steel sheet surface temperature is less than 40%, the effect of refining the magnetite in the scale layer cannot be obtained, and the eutectoid of magnetite and iron The proportion of the organization increases. For this reason, at the time of finish rolling, the appropriate range of the cumulative rolling reduction within the appropriate temperature range of the steel sheet surface temperature is set to 40% or more. In the present embodiment, the cumulative rolling reduction within the appropriate temperature range of the steel sheet surface temperature is more preferably 60% or more.
Note that the cumulative rolling reduction described in the present invention is the following formula when the initial thickness at the start of finish rolling is t0 and the thickness after finish rolling is tf for finish rolling performed within the above temperature range. This is the amount determined by (1).
Cumulative rolling reduction = {(t0−tf) / t0} × 100 (1)

On the other hand, if the finish rolling finish temperature is less than 800 ° C., the ferrite fraction increases, an appropriate yield ratio cannot be obtained, and the low-temperature toughness decreases. For this reason, in this embodiment, the suitable range of finish rolling end temperature is restrict | limited to 800 degreeC or more. In the present embodiment, the finish rolling end temperature is preferably 850 ° C. or higher from the viewpoint of ensuring a high yield ratio and excellent low temperature toughness.
In finish rolling, since roll rolling is usually performed a plurality of times, rolling treatment under other conditions is performed as long as it includes rolling with a cumulative rolling reduction of 40% or more within the temperature range of the steel sheet surface temperature. You may do.

  After finishing rolling that satisfies the above conditions, the hot-rolled steel sheet is cooled. If it is in this steel component range, the said cooling conditions will not be specifically limited, Each condition conventionally used can be employ | adopted.

Next, in the manufacturing method of the present embodiment, the winding temperature at the time of winding the steel strip that has been subjected to the finish rolling, together with the metal structure of the hot-rolled steel sheet, the volume fraction of magnetite and the magnetite grains in the scale layer It is an important factor to control the diameter and the proportion of eutectoid structure of magnetite and iron.
When the coiling temperature of the steel strip is less than 200 ° C., the martensite fraction increases and the low-temperature toughness decreases, and the yield ratio decreases. Further, when the coiling temperature is less than 200 ° C., transformation to magnetite does not occur sufficiently in the scale layer, so that good coating corrosion resistance cannot be obtained. On the other hand, when the coiling temperature of the steel strip exceeds 550 ° C., the ferrite fraction increases, the martensite and retained austenite fractions increase, and the yield ratio decreases. For this reason, in the manufacturing method of this embodiment, the appropriate range of the winding temperature of a steel strip is restrict | limited to the range of 200-550 degreeC. Note that the coiling temperature is desirably 500 ° C. or less from the viewpoint of making the particle size of the magnetite more appropriate.

Moreover, in this embodiment, after winding up within the range of the said coiling temperature and making it a coiled hot-rolled steel plate, you may reheat a hot-rolled steel plate between 150-500 degreeC. This reheating makes it possible to reduce the martensite fraction and obtain a higher yield ratio and better low temperature toughness.
When the reheating temperature is less than 150 ° C., the effect of lowering the martensite fraction is small, and when the reheating temperature exceeds 500 ° C., it becomes difficult to ensure a maximum tensile strength of 780 MPa or more. In addition, as a reheating temperature, 400 degreeC is a more desirable upper limit. The time for reheating and holding is not particularly limited, but is preferably 30 minutes or longer and 3 days or shorter.

In this embodiment, the volume fraction of magnetite in the scale layer is determined by measuring the surface of the hot-rolled steel sheet by X-ray diffraction, or by cross-sectioning the hot-rolled steel sheet by the EBSD method (electron beam backscattered electron diffraction method). You may measure by.
The average crystal grain size of magnetite can be obtained as a nominal grain size by measuring 100 or more crystal grains by the EBSD method in a cross section of a hot-rolled steel sheet.
The eutectoid structure of magnetite and iron has a lamellar shape, and the area fraction can be quantified by image analysis of a photograph taken with a scanning electron microscope.

According to the high yield ratio high strength hot-rolled steel sheet according to the present embodiment as described above, the scale layer and the ground can be formed by the above configuration even when electrodeposition coating is applied to the hot-rolled steel sheet having the scale layer. Adhesiveness with iron is not impaired, and a good chemical conversion coating can be formed, and excellent coating corrosion resistance and low temperature toughness can be obtained. As a result, when the conventional hot-rolled steel sheet is applied to a building machine part, the thickness of the part was set to allow for the amount of thinning due to corrosion. Since the rolled steel sheet has excellent paint corrosion resistance, it is possible to reduce the thickness of the parts to be applied, and as a result, the construction machine can be reduced in weight and lengthened.
In addition, according to the method for producing a high yield ratio high strength hot rolled steel sheet having excellent coating corrosion resistance and low temperature toughness according to the present invention, by adopting the above procedure and conditions, a high strength heat having excellent coating corrosion resistance and low temperature toughness. It becomes possible to manufacture a rolled steel sheet.

  Hereinafter, examples of the high yield ratio high strength hot-rolled steel sheet excellent in coating corrosion resistance and low temperature toughness according to the present invention will be described to explain the present invention more specifically, but the present invention is originally limited to the following examples. However, the present invention can be carried out with appropriate modifications within a range that can be adapted to the above and the gist described below, and these are all included in the technical scope of the present invention.

In this example, first, steel Nos. A to Y having the steel components shown in Table 1 below were cast, and then this slab was reheated and subjected to rough rolling.
Next, after changing the average remaining thickness of the scale layer using a descaling apparatus, finish rolling was performed under the conditions shown in Table 2 below. Thereafter, a winding process was performed at a predetermined temperature.
And the evaluation test which is demonstrated below was done about the hot-rolled steel sheet of the example of the present invention obtained by the above-mentioned procedure, and a comparative example.

First, the volume fraction of magnetite in the scale layer is quantified by the X-ray diffraction method, and the crystal grain size of the magnetite present in the scale layer is determined by separating the magnetite phase by the EBSD method. The particle size was measured. In addition, since the base iron and the iron in the scale layer cannot be distinguished by the X-ray diffraction method, there may be a difference from the fraction obtained by the cross-sectional observation method such as the EBSD method. In this case, the value obtained by the cross-sectional observation method is adopted.
Moreover, the block particle diameter of tempered martensite and bainite analyzed the part of 1/4 thickness of the steel plate full thickness by the EBSD method. Here, the block grain size is an aggregate of laths having the same orientation, and those having a crystal orientation difference of 5 ° or less between adjacent laths were regarded as the same orientation.

  In addition, the tensile properties of the hot-rolled steel sheet were obtained by taking JIS No. 5 test pieces from each hot-rolled steel sheet and performing the conditions so that the tensile direction was perpendicular to the rolling direction (C direction). The yield strength was measured. In addition, as for the yield strength, when the upper yield point was observed, the upper yield point strength was taken as the yield strength, and when the upper yield point was not seen, the flow stress at 0.2% elongation was taken as the yield strength. The yield ratio (YR) was determined from the obtained maximum tensile strength and yield strength.

Further, the low temperature toughness of the hot-rolled steel sheet was evaluated by a 2 mm V notch Charpy impact test using a test piece in the rolling direction according to the method described in JIS Z2242. In the present invention, the case where the absorbed energy vE- ₄₀ when the test temperature is −40 ° C. is 40 J / cm ² or more and the ductile brittle transition temperature vTrs is −20 ° C. or less is satisfactory (in Table 2, “◯ ").
The bendability is evaluated by a 90 ° V-bending test. The bending is performed when R / t is 0.5 or less, where R is the minimum punch tip radius where cracks did not occur and t is the thickness of the material. It was evaluated as having good properties (indicated as “◯” in Table 2).
Moreover, about the coating corrosion resistance after a shaping | molding process, first, 90 degree V bending process is performed with respect to the obtained hot-rolled steel plate (with a scale layer), Then, the said hot-rolled steel plate is degreased, Then, pre-processing After the zinc phosphate treatment (chemical conversion treatment), cationic electrodeposition coating was performed at a thickness of 25 μm. And after giving a linear wrinkle to the electrodeposition coating surface, according to the method of JISZ2371, the 200h salt spray test (SST test) is performed, and the coating film peeling at the time of performing a tape peeling test after this test The width was measured. Those having a coating peel width of 2 mm or less have good corrosion resistance (indicated as “◯” in Table 2), and those having a coating peel width exceeding 2 mm have poor corrosion resistance (“×” in Table 2). The evaluation was made in two stages. In the present invention, coating is not limited to electrodeposition baking coating, and other coating may be performed.

  Table 1 below shows a list of steel components, and Table 2 below shows the analysis results of the scale layer present in the produced hot-rolled steel sheet, tensile strength (TS), yield ratio (YR), bendability, low temperature toughness, A list of coating corrosion resistance evaluation results is shown. In Table 2 below, each heading indicates the following item.

SRT: Slab heating temperature t _scale : Average scale thickness at the start of finish rolling (mm)
Red: Cumulative rolling reduction between 820 and 980 ° C. (%)
FT: Final finish rolling temperature (° C)
CT: Winding temperature (° C)
RT: Reheating temperature after completion of winding (° C.)
d _B : block particle size (μm) of tempered martensite and bainite
f _F : Area fraction of ferrite (%)
f _{M + γ} : Area fraction (%) of the total of martensite and retained austenite
f _{TM + B} : Total area fraction (%) of tempered martensite and bainite
f _mag : volume fraction of magnetite in the scale layer (%)
d _mag : average particle diameter of magnetite (μm)
f _e : Area fraction (%) of the eutectoid structure of magnetite and iron in the scale layer

As shown in Table 2, the magnetite was produced under the conditions specified in the present invention, and the volume fraction of magnetite in the scale layer within the range specified by the present invention, the average crystal grain size of magnetite, and the eutectoid of magnetite and iron In the hot rolled steel sheets of the present invention examples in which the area fraction of the structure and the metal structure are controlled, TS is 780 MPa or more, the yield ratio is 0.85 or more, and vE- ₄₀ is 50 J or more, Moreover, the evaluation of the bendability and the paint corrosion resistance of the bent portion was “◯”. Thereby, it became clear that the high yield ratio high strength hot rolled steel sheet of the present invention is excellent in coating corrosion resistance and low temperature toughness.

  On the other hand, the hot-rolled steel sheet of the comparative example shown in Table 2 has a volume fraction of magnetite in the scale layer, an average crystal grain size of magnetite, an area fraction of the eutectoid structure of magnetite and iron, and a metal structure. Since any of these did not satisfy the specified range of the present invention, at least one of bending workability, coating corrosion resistance, and low temperature toughness was inferior.

Test number A-2 is an example in which the low temperature toughness was lowered because the heating temperature of the slab was higher than the appropriate range, while test number A-3 was sufficient because the heating temperature of the slab was lower than the appropriate range. This is an example in which a high tensile strength could not be obtained and the low temperature toughness was also lowered.
Test No. A-4 is an example in which descaling was performed excessively and the average scale thickness at the start of finish rolling was small, so that the magnetite crystal grains were large and the coating corrosion resistance was evaluated as poor.
Test number A-6 is an example in which the average scale thickness at the start of finish rolling was excessive compared to the specified range of the present invention, so that the magnetite fraction was reduced and the coating corrosion resistance was evaluated as poor. is there.
In addition, although test numbers A-8 and B-2 had an appropriate average scale thickness at the start of finish rolling, no strain was imparted to the scale layer during finish rolling (the cumulative rolling reduction was small). This is an example in which the magnetite crystal grains are not refined, the eutectoid structure fraction of magnetite and iron is excessive, and the coating corrosion resistance is poor.
In Test No. A-9, since the final finish rolling temperature was low, the scale layer was broken, the magnetite fraction was reduced, the coating corrosion resistance was poor, the ferrite fraction was excessive, and the yield ratio was lowered. Further, this is an example in which the low temperature toughness is lowered.
Moreover, test number A-10 is an example in which the coiling temperature was above the appropriate range, the ferrite fraction was excessive, the yield ratio was reduced, and the low temperature toughness was further reduced.
In Test No. A-12, since the coiling temperature was less than the appropriate range, the transformation from wustite to magnetite did not occur sufficiently, resulting in poor corrosion resistance and further reduced martensite fraction and yielding. This is an example in which the ratio falls below the appropriate range.

In addition, test numbers F-1, I-1, J-1, and S-1 are not suitable steel components, and accordingly, the magnetite fraction and the particle size of the magnetite are outside the proper ranges, and the coating corrosion resistance is poor. This is an example.
In addition, test numbers F-1, G-1, H-1, K-1, L-1, M-1, N-1, O-1, P-1, Q-1, R-1, S- 1, T-1, U-1, and V-1 are examples in which the steel component was not appropriate, and thus the maximum tensile strength was less than 780 MPa, and the yield ratio, low temperature toughness, and bendability were reduced.

  From the results of the examples described above, the hot-rolled steel sheet of the present invention has a maximum tensile strength of 780 MPa or more, further has a yield ratio of 0.85 or more, and is formed into a hot-rolled steel sheet according to the present invention having a scale layer. Even when the electrodeposition baking coating is applied after the coating, it is clear that the coating is excellent in corrosion resistance after coating, and further excellent in low temperature toughness and bending workability.

  The high yield ratio high strength hot-rolled steel sheet shown in the present invention is used for a construction machine member such as a crane boom, for example, to reduce the weight of the member itself and increase the lifting and carrying capacity. Its social contribution is immeasurable because it can fully enjoy the benefits of significantly improved efficiency.

Claims (6)

% By mass
C: 0.05 to 0.15%,
Si: 0.4% or less,
Al: 0.4% or less,
Mn: 1.2 to 2.5%
P: 0.1% or less,
S: 0.01% or less,
N: 0.007% or less,
Ti: 0.03-0.14%,
Nb: 0.008 to 0.06%
B: 0.0003 to 0.0030%
Containing, and
2Mo + Cr: 0.2 to 1.0%
And the balance has a steel component consisting of Fe and inevitable impurities,
Steel structure is bainite and tempered martensite with a total area ratio of 90% or more, martensite and retained austenite with a total area ratio of 5% or less, and ferrite with an area ratio of 10% or less,
And the block particle size of the said bainite and the said tempered martensite is 5 micrometers or less,
Further, the volume fraction of magnetite in the scale layer formed on the surface layer of the steel sheet is 60% or more, the average crystal grain size of the magnetite is 3 μm or less, and the magnetite and iron are co-located in the scale layer. A high-yield-ratio high-strength hot-rolled steel sheet characterized in that the area fraction of the deposited structure is 40% or less, the maximum tensile strength is 780 MPa or more, and the yield ratio is 0.85 or more. The high yield ratio high strength hot-rolled steel sheet according to claim 1 , wherein the steel component further contains V: 0.01 to 0.12% by mass. The steel component according to any one of claims 1 and 2 , further comprising 0.02 to 2.0% in total of at least one of Cu and Ni in terms of mass%. High yield ratio high strength hot rolled steel sheet. In the steel components, further containing, by mass%, Ca, Mg, La, claims 1 to 3, characterized in that it contains 0.0003 to 0.01% in total of one or more of Ce The high yield ratio high strength hot-rolled steel sheet according to any one of the above. It is a method of manufacturing the high-strength hot-rolled steel sheet according to any one of claims 1 to 4 , wherein slab heating is performed at 1050 to 1200 ° C, and an average thickness of the scale layer at the start of finish rolling is After descaling to 3 to 30 μm, finish rolling is then performed with the steel sheet surface temperature in the range of 820 to 980 ° C., the cumulative rolling reduction of 40% or more, and the finish rolling finish temperature of 800 ° C. or more. Then, a method for producing a high yield ratio high strength hot-rolled steel sheet, which is wound in a coil shape with a coiling temperature in the range of 200 to 550 ° C. The method for producing a high yield ratio high strength hot rolled steel sheet according to claim 5 , wherein after the winding, the coiled hot rolled steel sheet is reheated to a temperature range of 150 to 500 ° C.