WO2015033933A1

WO2015033933A1 - Hot-rolled steel sheet having excellent cold workability and excellent surface properties and hardness after working

Info

Publication number: WO2015033933A1
Application number: PCT/JP2014/073075
Authority: WO
Inventors: 梶原　桂; ▲琢▼哉高知
Original assignee: 株式会社神戸製鋼所
Priority date: 2013-09-04
Filing date: 2014-09-02
Publication date: 2015-03-12
Also published as: DE112014004028T5; US20160201172A1; JP2015048527A; JP6058508B2; MX2016002690A; CN105492645A

Abstract

A hot-rolled steel sheet which has a thickness of 3-20 mm and contains specific amounts of C, Si, Mn, P, S, Al and N, with the balance made up of iron and unavoidable impurities. This hot-rolled steel sheet contains a specific amount of solid-solved N, and the contents of C and N satisfy the relation 10C + N ≤ 3.0. This hot-rolled steel sheet contains bainitic ferrite and pearlite, respectively in an area ratio of 5% or more and in an area ratio of less than 20% relative to the whole structure, with the balance made up of polygonal ferrite. The average crystal grain size of the bainitic ferrite is 3-50 μm.

Description

Hot-rolled steel sheet with excellent cold workability, surface properties and hardness after processing

The present invention relates to a hot-rolled steel sheet that exhibits good cold workability during processing, and exhibits predetermined surface properties (also referred to as “surface quality”) and post-working hardness after processing.

In recent years, from the viewpoint of environmental protection, for the purpose of improving the fuel efficiency of automobiles, there is an increasing demand for reducing the weight of steel materials used for various parts for automobiles, for example, transmission parts such as gears and cases, that is, increasing the strength. . In order to meet such demands for weight reduction and high strength, steel materials obtained by hot forging steel bars (hot forging materials) have been used as steel materials that are generally used. In addition, in order to reduce CO ₂ emissions in the component manufacturing process, there is an increasing demand for cold forging of components such as gears that have been processed by hot forging.

By the way, cold working (cold forging) has advantages of higher productivity than hot working and warm working and good dimensional accuracy and yield of steel materials. However, the problem in manufacturing parts by such cold working is that in order to ensure the strength of the cold-worked parts to be higher than the expected value, the strength, ie deformation, is inevitably required. It is necessary to use a steel material with high resistance. However, the higher the deformation resistance of the steel material used, the shorter the service life of the cold working mold.

In the field of transmission parts, we are also considering the manufacture of parts from steel forgings (hot forging, cold forging, etc.) using steel sheets with the aim of reducing the weight and cost of parts. In the parts after the inter-working, there is a difficulty that surface defects called stretcher strain marks (hereinafter abbreviated as “SS marks”) are likely to occur on the surface.

For this reason, conventionally, after cold forging a steel material into a predetermined shape, a method of manufacturing a high-strength part with a predetermined strength (hardness) is performed by performing a heat treatment such as quenching and tempering. There was also. However, since heat treatment after cold forging inevitably changes the part dimensions, it is necessary to correct by secondary machining such as cutting, and a solution that can omit heat treatment and subsequent machining is desired. It was rare.

In order to solve the above-mentioned problem, for example, by using solute C in low carbon steel, the progress of normal temperature aging is suppressed, and a predetermined age hardening amount due to strain aging is ensured, thereby providing a cold having excellent strain aging characteristics. It is disclosed that a wire rod and steel bar for forging can be obtained (see Patent Document 1).

However, this technology controls strain aging only by the amount of solute C, and it is difficult to obtain a steel material having sufficient cold workability and required surface quality, hardness and strength after processing. Met.

Therefore, the present applicant paid attention to the difference in the effects of solute C and solute N contained in steel materials on deformation resistance and static strain aging, and as a result of various studies, the amount of these solute elements was appropriately determined. By controlling, it is found that a steel material for machine structure showing a predetermined hardness (strength) can be obtained after cold working (cold forging) while exhibiting good cold workability during working, A patent application has already been filed (see Patent Document 2).

This steel material realizes both cold workability and high hardness (high strength) after processing, but is a hot forging material, like the wire rod and bar steel described in Patent Document 1 above. The manufacturing cost is high. Therefore, in order to further reduce the manufacturing cost, it has been studied to produce automobile parts by cold working using hot-rolled steel sheets instead of the conventional hot forging materials.

For example, a hot-rolled steel sheet for nitriding that has a high surface hardness and a sufficient hardening depth after nitriding has been proposed (see Patent Document 3).

However, this technique requires further nitriding after cold working, and there is a problem that a sufficient cost reduction cannot be realized.

Further, it contains C: 0.10% or less, Si: less than 0.01%, Mn: 1.5% or less, and Al: 0.20% or less, and (Ti + Nb) / 2: 0.05-0. It is contained in the range of 50%, S: 0.005% or less, N: 0.005% or less, O: 0.004% or less, the total of S, N and O is 0.0100% or less, In addition, a hot-rolled steel sheet having a microstructure of 95% or more of a substantially ferritic single-phase structure has been proposed. Is extremely high, and is also excellent in red scale resistance (see Patent Document 4).

However, in this hot-rolled steel sheet, N is limited to a very low content as a harmful element, and the technical idea is completely different from the hot-rolled steel sheet according to the present invention that actively uses N. It is.

Japanese Unexamined Patent Publication No. 10-306345 Japanese Unexamined Patent Publication No. 2009-228125 Japanese Unexamined Patent Publication No. 2007-162138 Japanese Unexamined Patent Publication No. 2004-137607

The present invention has been made by paying attention to the above circumstances, and its object is to provide a hot-rolled steel sheet that exhibits a good cold workability during processing and exhibits a predetermined surface property and hardness after processing. There is.

The invention described in claim 1
The plate thickness is 3-20mm,
Ingredient composition
% By mass (hereinafter the same for chemical components)
C: 0.3% or less (excluding 0%),
Si: 0.5% or less (excluding 0%),
Mn: 0.2 to 1%
P: 0.05% or less (excluding 0%),
S: 0.05% or less (excluding 0%),
Al: 0.01 to 0.1%,
N: 0.008 to 0.025%,
The balance consists of iron and inevitable impurities,
Solid solution N: 0.007% or more, and
The content of C and N satisfies the relationship of 10C + N ≦ 3.0,
Organization
The area ratio for all tissues
Bainitic ferrite: 5% or more
Perlite: less than 20%
The rest: polygonal ferrite,
A hot rolled steel sheet excellent in cold workability, surface properties and hardness after processing, characterized in that the average crystal grain size of the bainitic ferrite is in the range of 3 to 50 μm.

The invention described in claim 2
The hot rolled steel sheet according to claim 1, wherein the component composition further contains at least one of the following (a) to (e).
(A) At least one of Cr: 2% or less (not including 0%) and Mo: 2% or less (not including 0%) (b) Ti: 0.2% or less (not including 0%), Nb : 0.2% or less (excluding 0%), V: at least one selected from the group consisting of 0.2% or less (not including 0%) (c) B: 0.005% or less (0% Not included)
(D) At least one selected from the group consisting of Cu: 5% or less (not including 0%), Ni: 5% or less (not including 0%), Co: 5% or less (not including 0%)
(E) Ca: 0.05% or less (not including 0%), REM: 0.05% or less (not including 0%), Mg: 0.02% or less (not including 0%), Li: 0.02% or less (not including 0%), Pb: 0.5% or less (not including 0%), Bi: 0.5% or less (not including 0%) seed

According to the present invention, in a structure mainly composed of bainitic ferrite and polygonal ferrite having a predetermined average particle diameter, a solid solution N amount is ensured, and the C content and the N content have a predetermined relationship. By satisfying, the deformation resistance during cold working is reduced, the life of the mold is extended, the steel plate is less likely to crack, and the parts obtained after processing have the prescribed surface properties and hardness after processing It is now possible to provide a hot-rolled steel sheet that can ensure the above.

Hereinafter, the hot-rolled steel sheet according to the present invention (hereinafter also referred to as “the steel sheet of the present invention” or simply “the steel sheet”) will be described in more detail. The steel sheet of the present invention is common to the hot forging material described in Patent Document 2 above in that the N solid solution amount is ensured and the C content and the N content satisfy a predetermined relationship. The difference is that the C content is allowed to a higher range, the structure is a bainitic ferrite-polygonal ferrite-pearlite double phase structure, and the bainitic ferrite grains are refined.

[Thickness of the steel sheet of the present invention: 3 to 20 mm]
First, the steel sheet of the present invention has a thickness of 3 to 20 mm. If the plate thickness is less than 3 mm, rigidity as a structure cannot be secured. On the other hand, if the plate thickness exceeds 20 mm, it is difficult to achieve the tissue form defined in the present invention, and the desired effect cannot be obtained. A preferred plate thickness is 4 to 19 mm.

Next, the component composition constituting the steel sheet of the present invention will be described. Hereinafter, all the units of chemical components are mass%.

[Component composition of the steel sheet of the present invention]
<C: 0.3% or less (excluding 0%)>
C is an element that has a great influence on the formation of the structure of a steel sheet, and the structure is bainitic ferrite-polygonal ferrite-pearlite multiphase structure, but bainitic ferrite-polygonal ferrite mainly containing as little pearlite as possible. It is an element whose content needs to be limited in order to form a structure. When C is contained excessively, the pearlite fraction in the steel sheet structure increases, and the deformation resistance may be excessive due to work hardening of the pearlite. Therefore, the C content in the steel sheet is limited to 0.3% or less, preferably 0.25% or less, more preferably 0.2% or less, and particularly preferably 0.15% or less. However, if the content of C is too small, deoxidation during the melting of steel becomes difficult, and it becomes difficult to satisfy the strength and hardness after cold working, so 0.0005% or more, more preferably Is 0.0008% or more, particularly preferably 0.001% or more.

<Si: 0.5% or less (excluding 0%)>
Si is an element that needs to be reduced as much as possible in order to increase the deformation resistance of the steel sheet by dissolving in steel. Therefore, the Si content in the steel sheet is 0.5% or less, preferably 0.45% or less, more preferably 0.4% or less, and particularly preferably 0.3% or less in order to suppress an increase in deformation resistance. Restrict to. However, if the Si content is extremely small, deoxidation during melting becomes difficult, and it becomes difficult to satisfy the strength and hardness after cold working, so 0.005% or more, more preferably 0.008% or more, particularly preferably 0.01% or more.

<Mn: 0.2-1%>
Mn is an element having a deoxidizing and desulfurizing action in the steel making process. Furthermore, when the content of N in the steel material is increased, cracks are likely to occur due to dynamic strain aging due to heat generation during processing. On the other hand, Mn improves the workability at that time and has the effect of suppressing cracks. . In order to effectively exhibit these actions, the Mn content in the steel material is 0.2% or more, preferably 0.22% or more, and more preferably 0.25% or more. However, if the Mn content is excessive, deformation resistance becomes excessive and nonuniformity of the structure due to segregation occurs. Therefore, it is 1% or less, preferably 0.98% or less, and more preferably 0.95% or less.

<P: 0.05% or less (excluding 0%)>
P is an impurity element inevitably contained in the steel, but if it is contained in ferrite, it segregates at the ferrite grain boundaries and degrades the cold workability. It is an element that causes an increase. Therefore, it is desirable to reduce the P content as much as possible from the viewpoint of cold workability. However, since extreme reduction leads to an increase in steelmaking costs, it is 0.05% or less, preferably in consideration of process capability. 0.03% or less.

<S: 0.05% or less (excluding 0%)>
S, like P, is an unavoidable impurity and is an element that precipitates in the form of a film at the grain boundary as FeS and degrades workability. It also has the effect of causing hot brittleness. Therefore, from the viewpoint of improving the deformability, in the present invention, the S content is set to 0.05% or less, preferably 0.03% or less. However, it is industrially difficult to reduce the S content to zero. In addition, since S has an effect of improving machinability, it is recommended to contain 0.002% or more, more preferably 0.006% or more from the viewpoint of improving machinability.

<Al: 0.01 to 0.1%>
Al is an element effective for deoxidation in the steelmaking process. In order to obtain this deoxidation effect, the Al content in the steel material is set to 0.01% or more, preferably 0.015% or more, and more preferably 0.02% or more. However, if the Al content is excessive, the toughness is lowered and cracking is likely to occur. Therefore, the Al content is 0.1% or less, preferably 0.09% or less, and more preferably 0.08% or less.

<N: 0.008 to 0.025%>
N is an important element for obtaining a predetermined strength by static strain aging after processing. Therefore, the N content in the steel material is 0.008% or more, preferably 0.0085% or more, and more preferably 0.009% or more. However, if the N content is excessive, in addition to static strain aging, the influence of dynamic strain aging during processing becomes significant, and deformation resistance increases, which is unsuitable. Therefore, 0.025% or less, preferably 0 0.02% or less, more preferably 0.02% or less.

<Solution N: 0.007% or more>
By securing a predetermined amount of solid solution N in the steel sheet (hereinafter referred to as “solid solution N amount”), the static strain aging can be promoted without significantly increasing the deformation resistance. In order to ensure the required strength after cold working, the amount of solute N needs to be 0.007% or more. However, if the amount of solute N is excessive, the cold workability is deteriorated and the amount of solute N fixed to the processing strain is also increased, so that SS marks are easily generated and the surface properties are also deteriorated. Is 0.03% or less. In addition, since content of N in steel materials is 0.025% or less, the amount of solute N does not become 0.025% or more substantially.

Here, the solid solution N amount in the present invention is an amount obtained by subtracting the amount of all N compounds from the total N amount in the steel material in accordance with JIS G 1228. A practical method for measuring the amount of dissolved N will be exemplified below.

(A) Inert gas melting method-thermal conductivity method (measurement of total N content)
A sample cut from the test material is put in a crucible, extracted in an inert gas stream to extract N, the extract is transported to a thermal conductivity cell, and the change in thermal conductivity is measured to determine the total N amount. Ask.
(B) Ammonia distillation separation indophenol blue spectrophotometry (measurement of total N compound amount)
A sample cut out from the test material is dissolved in a 10% AA-based electrolytic solution, subjected to constant current electrolysis, and the total amount of N compounds in the steel is measured. The 10% AA electrolyte used is a non-aqueous solvent electrolyte consisting of 10% acetone, 10% tetramethylammonium chloride, and the remainder methanol, and does not generate a passive film on the steel surface.

About 0.5 g of the sample material is dissolved in this 10% AA-based electrolyte, and the resulting insoluble residue (N compound) is filtered through a polycarbonate filter having a hole size of 0.1 μm. The obtained insoluble residue is decomposed by heating in a chip made of sulfuric acid, potassium sulfate and pure copper, and the decomposition product is combined with the filtrate. After making this solution alkaline with sodium hydroxide, steam distillation is performed, and the distilled ammonia is absorbed in dilute sulfuric acid. Further, phenol, sodium hypochlorite and sodium pentacyanonitrosyl iron (III) are added to form a blue complex, and the absorbance is measured using an absorptiometer to determine the total N compound amount.
And the amount of solid solution N can be calculated | required by subtracting the total N compound amount calculated | required by the method of said (b) from the total N amount calculated | required by the method of said (a).

<The content of C and N satisfies the relationship of 10C + N ≦ 3.0>
In the steel material of the present invention, solid solution C greatly increases deformation resistance and does not contribute much to static strain aging, while solid solution N promotes static strain aging without significantly increasing deformation resistance. Therefore, the hardness after processing can be increased. Therefore, in the steel material of the present invention, in order to increase the hardness after processing without significantly increasing the deformation resistance during processing, the content of C and the content of N are 10C + N ≦ 3.0. It is essential to satisfy the relationship, preferably 0.009 ≦ 10C + N ≦ 2.8, more preferably 0.01 ≦ 10C + N ≦ 2.5, and particularly preferably 0.01 ≦ 10C + N ≦ 2.0. From the viewpoint of refining crystal grains in a hot-rolled steel sheet and ensuring the formability of the steel sheet, a C content and a solute C content are required to some extent, but when 10C + N> 3.0, C and / or N The amount becomes excessive and the deformation resistance becomes excessive. Here, in the above inequality, the coefficient of the C content is set to 10 times the coefficient of the N content because the solid solution C has the same content as the solid solution N but the strength in the hot-rolled steel sheet of the present invention. Further, it is considered that the degree of increasing the deformation resistance is about one digit (10 times) larger.

The steel of the present invention basically contains the above components, and the balance is iron and inevitable impurities, but the following permissible components can be added as long as the effects of the present invention are not impaired.

<Cr: 2% or less (not including 0%) and / or Mo: 2% or less (not including 0%)>
Cr is an element that has the effect of improving the deformability of steel by increasing the strength of the grain boundaries. In order to effectively exhibit such an effect, Cr is preferably contained in an amount of 0.2% or more. . However, if Cr is excessively contained, deformation resistance increases and cold workability may be lowered. Therefore, the content is recommended to be 2% or less, more preferably 1.5% or less, especially 1% or less. Is done.

Mo is an element having an action of increasing the hardness and deformability of the steel material after processing. In order to effectively exhibit such action, Mo is 0.04% or more, more preferably 0. It is preferable to contain 0.08% or more. However, if Mo is excessively contained, the cold workability may be deteriorated. Therefore, the content is recommended to be 2% or less, further 1.5% or less, particularly 1% or less.

<Ti: 0.2% or less (excluding 0%),
Nb: 0.2% or less (excluding 0%),
V: at least one selected from the group consisting of 0.2% or less (excluding 0%)>
These elements have a strong affinity for N, coexist with N to form N compounds, refine steel grains, improve the toughness of processed products obtained after cold working, and improve crack resistance. It is an element that has a role to improve. However, even if each element is contained exceeding the upper limit value, the effect of improving the characteristics cannot be obtained. It is recommended that the content of each element is 0.2% or less, further 0.001 to 0.15%, particularly 0.002 to 0.1%.

<B: 0.005% or less (excluding 0%)>
B, like Ti, Nb, and V, has a strong affinity with N, and coexists with N to form an N compound, refines the grain of steel, and improves the toughness of the workpiece obtained after cold working And an element having a role of improving crack resistance. Therefore, when the steel sheet of the present invention contains B, the required solid solution N amount can be secured and the strength after cold working can be improved, so the content is 0.005% or less, and further 0 0.0001 to 0.0035%, especially 0.0002 to 0.002% is recommended.

<Cu: 5% or less (excluding 0%),
Ni: 5% or less (excluding 0%),
Co: at least one selected from the group consisting of 5% or less (excluding 0%)>
All of these elements have the effect of strain aging and hardening the steel material, and are effective in improving the strength after processing. In order to effectively exhibit such an action, these elements are preferably contained in an amount of 0.1% or more, and more preferably 0.3% or more. However, if the content of these elements is excessive, the effects of strain aging and hardening of the steel material, and the effect of improving the strength after processing are saturated, and there is a possibility of promoting cracking. In the following, 4% or less, particularly 3% or less is recommended.

<Ca: 0.05% or less (excluding 0%),
REM: 0.05% or less (excluding 0%),
Mg: 0.02% or less (excluding 0%),
Li: 0.02% or less (excluding 0%),
Pb: 0.5% or less (excluding 0%),
Bi: at least one selected from the group consisting of 0.5% or less (excluding 0%)>
Ca is an element that spheroidizes sulfide compound inclusions such as MnS, improves the deformability of steel, and contributes to improvement of machinability. In order to effectively exhibit such an action, Ca is preferably contained in an amount of 0.0005% or more, more preferably 0.001% or more. However, even if contained excessively, the effect is saturated and an effect commensurate with the content cannot be expected. Therefore, 0.05% or less, further 0.03% or less, particularly 0.01% or less is recommended.

REM is an element that, like Ca, spheroidizes compound inclusions such as MnS to increase the deformability of steel and contribute to the improvement of machinability. In order to effectively exhibit such an action, REM is preferably contained in an amount of 0.0005% or more, more preferably 0.001% or more. However, even if contained excessively, the effect is saturated and an effect commensurate with the content cannot be expected. Therefore, 0.05% or less, further 0.03% or less, particularly 0.01% or less is recommended.
In the present invention, REM means a lanthanoid element (15 elements from La to Lu), Sc (scandium) and Y (yttrium). Among these elements, it is preferable to contain at least one element selected from the group consisting of La, Ce and Y, more preferably La and / or Ce.

Mg, like Ca, is an element that spheroidizes sulfide compound inclusions such as MnS to enhance the deformability of steel and contribute to the improvement of machinability. In order to effectively exhibit such an action, Mg is preferably contained in an amount of 0.0002% or more, more preferably 0.0005% or more. However, even if contained excessively, the effect is saturated and an effect commensurate with the content cannot be expected, so 0.02% or less, further 0.015% or less, and particularly 0.01% or less is recommended.

Li can spheroidize sulfide compound inclusions such as MnS and increase the deformability of steel like Ca, and lower the melting point of Al-based oxides to make them harmless and improve machinability. It is a contributing element. In order to effectively exhibit such an action, Li is preferably contained in an amount of 0.0002% or more, and more preferably 0.0005% or more. However, even if contained excessively, the effect is saturated and an effect commensurate with the content cannot be expected, so 0.02% or less, further 0.015% or less, and particularly 0.01% or less is recommended.

Pb is an effective element for improving machinability. In order to effectively exhibit such an action, Pb is preferably contained in an amount of 0.005% or more, and more preferably 0.01% or more. However, if it is contained excessively, production problems such as generation of rolling defects occur, so 0.5% or less, further 0.4% or less, particularly 0.3% or less is recommended.

Bi is an element effective for improving the machinability like Pb. In order to effectively exhibit such an action, Bi is preferably contained in an amount of 0.005% or more, and more preferably 0.01% or more. However, since the effect of improving the machinability is saturated even if contained excessively, 0.5% or less, further 0.4% or less, and particularly 0.3% or less are recommended.

Next, the structure characterizing the steel sheet of the present invention will be described.

[Structure of the steel sheet of the present invention]
As described above, the steel sheet of the present invention is based on bainitic ferrite-polygonal ferrite pearlite double phase steel, and is particularly characterized by controlling the bainitic ferrite grain size within a specific range. To do.

<Bainitic ferrite: 5% or more, pearlite: less than 20%, balance: polygonal ferrite>
The structure of the steel sheet of the present invention is composed of a multiphase structure of bainitic ferrite, polygonal ferrite and pearlite. Bainitic ferrite has the effect of improving the workability during cold working and increasing the hardness after processing while suppressing the occurrence of stretcher strain marks. The area ratio is 5% or more, preferably 10% or more, and more preferably 15% or more. Moreover, since the formability of a steel plate is deteriorated when excessive pearlite is present, the pearlite is less than 20% in area ratio, more preferably 19% or less, still more preferably 18% or less, and particularly preferably 15% or less. The balance is polygonal ferrite.
In addition to the above structure, a cementite phase is also present in the structure of the steel sheet of the present invention, but the area ratio is at most 1% or less, so in this specification, The area ratios of tick ferrite, polygonal ferrite, and pearlite were defined as those normalized so that the total area ratio of these three phases was 100%.

<Average crystal grain size of the bainitic ferrite: in the range of 3 to 50 μm>
The average grain size of bainitic ferrite constituting the bainitic ferrite structure needs to be in the range of 3 to 50 μm in order to improve the workability of the steel sheet and satisfy the surface properties after processing. . If the bainitic ferrite grains become too fine, the deformation resistance becomes too high, so the average crystal grain size is 3 μm or more, preferably 4 μm or more, more preferably 5 μm or more. On the other hand, if the bainitic ferrite is too coarse, the surface properties after processing deteriorate, and the toughness and fatigue characteristics deteriorate, so the average crystal grain size is 50 μm or less, preferably 45 μm or less, more preferably 40 μm. The following.

[Measurement method of area ratio of each phase]
Regarding the area ratio of each of the above phases, each test steel sheet was subjected to Nital corrosion, taken with a scanning electron microscope (SEM; magnification 1000 times), five fields of view, and the ratios of bainitic ferrite, polygonal ferrite and pearlite were pointed out. It can be obtained by arithmetic.
Here, bainitic ferrite is a ferrite grain that exists in the structure of bainite (generically referred to as upper bainite and lower bainite). Definition ”-understanding the current situation-heat treatment volume 50 No. 1, February 2010, p. 22-27), defined as having an aspect ratio (major axis / minor axis ratio) of 2 or more . Polygonal ferrite is defined as ferrite grains having equiaxed crystal grains and an aspect ratio (major axis / minor axis ratio) of less than 2.

[Measurement method of average crystal grain size]
The average crystal grain size of the bainitic ferrite can be measured as follows. That is, the crystal grain size of bainitic ferrite existing at three locations, the outermost layer portion, the plate thickness ¼ portion, and the plate thickness center portion, is measured. As for the particle size of one bainitic ferrite particle, the side part in the rolling direction of each measurement point was subjected to nital corrosion, and the corresponding part was photographed with five fields of view with a scanning electron microscope (SEM; magnification 1000 times). The average crystal grain size of ferrite crystal grains was determined based on the center of gravity diameter by image analysis.

Next, a preferred manufacturing method for obtaining the steel sheet of the present invention will be described below.

[Preferred production method of the steel sheet of the present invention]
The steel plate of the present invention may be produced according to any method as long as the raw steel having the above composition can be formed into a desired plate thickness. For example, it can be carried out by preparing a molten steel having the above component composition in a converter under the conditions shown below, slab this by ingot casting or continuous casting, and then rolling it into a hot-rolled steel sheet having a desired thickness. .

[Preparation of molten steel]
The N content in the molten steel is adjusted by adding a raw material containing an N compound to the molten steel and / or controlling the converter atmosphere to an N ₂ atmosphere during melting in the converter. can do.

[heating]
Heating before hot rolling is performed at 1100 to 1300 ° C. This heating requires high-temperature heating conditions in order to dissolve as much N as possible without producing an N compound. The minimum with a preferable heating temperature is 1100 degreeC, and a more preferable minimum is 1150 degreeC. On the other hand, temperatures exceeding 1300 ° C. are difficult to operate.

[Hot rolling]
Hot rolling is performed so that the finish rolling temperature is 880 ° C. or higher. If the finish rolling temperature is too low, ferrite transformation will occur at a high temperature, and the precipitated carbides in ferrite (generically referred to as bainitic ferrite and polygonal ferrite) will become coarse and fatigue strength will deteriorate. The above finish rolling temperature is required. The finish rolling temperature is more preferably 900 ° C. or higher in order to coarsen the austenite grains and increase the grain size of the bainitic ferrite to some extent. The upper limit of the finish rolling temperature is set to 1000 ° C. because it is difficult to secure the temperature.

The thickness of the hot-rolled steel sheet of the present invention is 3 to 20 mm. In order to refine the bainitic ferrite crystal grains and control the average crystal grain size within a predetermined grain size range, In addition to control, it is necessary to set the final reduction ratio of tandem rolling of finish rolling to 15% or more. Normally, the finish rolling is performed by tandem rolling of 5 to 7 passes, but a pass schedule is set from the viewpoint of control of sheet penetration, and the final rolling reduction is about 12 to 13%. The final rolling reduction is preferably 16% or more, more preferably 17% or more. The higher the final reduction ratio is 20% or 30%, the more effective the crystal grains are refined, but the upper limit is defined to be about 30% from the viewpoint of rolling control.

[Rapid cooling after hot rolling]
After finishing the finish rolling, quenching is performed at a cooling rate (first quenching rate) of 20 ° C./s or more within 5 s, and rapid cooling is stopped at a temperature of 550 ° C. or more and less than 650 ° C. (quenching stop temperature). This is for obtaining a bainitic ferrite-polygonal ferrite-pearlite double phase structure having a predetermined phase fraction. When the cooling rate (quenching rate) is less than 20 ° C./s, the pearlite transformation is promoted, or when the quenching stop temperature is less than 550 ° C., the bainite transformation is suppressed, both of which are bainitic ferrite-polygonal having a predetermined phase fraction. It becomes difficult to obtain ferrite-pearlite steel, and cold workability and surface quality after processing deteriorate. On the other hand, when the quenching stop temperature is 650 ° C. or more, the precipitated carbide in the ferrite is coarsened, and the fatigue strength is deteriorated. The quenching stop temperature is preferably 560 to 640 ° C, more preferably 580 to 620 ° C.

[Slow cooling after rapid cooling stop]
After the rapid cooling is stopped, it is slowly cooled for 5 to 20 seconds at a cooling rate (slow cooling rate) of 10 ° C./s or less by cooling or air cooling. Thereby, the formation of polygonal ferrite is sufficiently advanced, and the precipitated carbide in the ferrite is appropriately refined. When the cooling rate exceeds 10 ° C./s or the slow cooling time is less than 5 s, the amount of polygonal ferrite formed is insufficient. On the other hand, if the slow cooling time exceeds 20 s, the precipitated carbide is not coarsened and the fatigue strength is deteriorated.

[Rapid cooling after slow cooling, winding]
After the slow cooling, it is rapidly cooled again at a cooling rate (second quenching rate) of 20 ° C./s or more, and wound at 500 to 600 ° C. This is to ensure cold workability by making the structure mainly bainitic ferrite + ferrite. When the cooling rate (second rapid cooling rate) is less than 20 ° C./s or the coiling temperature exceeds 600 ° C., a large amount of pearlite is formed and the cold workability is deteriorated. As a result, the surface quality after processing is deteriorated.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and may be implemented with appropriate modifications within a range that can meet the purpose described above and below. These are all possible and are within the scope of the present invention.

Steel having the component composition shown in the following Table 1 was melted by a vacuum melting method, cast into an ingot having a thickness of 120 mm, and hot rolled under the conditions shown in Table 2 to produce a hot rolled steel sheet. In any test, the cooling rate until the quenching stop after finishing rolling is 20 ° C./s or more, and the cooling after the quenching stop is a condition of slow cooling for 5 to 20 s at a cooling rate of 10 ° C./s or less. Met.

About the hot-rolled steel sheet obtained in this way, the amount of solute N, the area ratio of each phase of the structure in the steel sheet, and the average crystal grain size of bainitic ferrite are as described above. It was determined by each measurement method described above.

Also, the hot-rolled steel sheet was evaluated for cold workability, surface quality after processing and hardness as follows.

(Evaluation of cold workability)
The cold workability was evaluated by the work hardening amount defined by the difference in hardness of the sample center before and after processing. Therefore, first, with respect to the hot-rolled steel sheet, using a processing reproducibility test apparatus (Thermecaster Z, manufactured by Fuji Radio Engineering Co., Ltd.), conditions of processing temperature: room temperature, rolling reduction: 70%, strain rate: 10 / s A processing test was conducted. The shape of the processed test piece was adjusted so that the diameter / height ratio was substantially constant, such as φ8 mm × 12 mm, φ6 mm × 10 mm, φ4 mm × 6 mm, according to the plate thickness. Hardness is measured using a Vickers hardness tester, and the measurement position is a center part in the compression direction of the processed test piece and is 1/4 part in the diameter direction of the circle (position between the center and the center of the outer circumference), load: 500 g, Number of measurements: Under the conditions of 5 times, the Vickers hardness (Hv) was measured for each of the work specimens before and after the work test, and the average of each was taken as the pre-work hardness and post-work hardness.

Then, as described above, the cold workability was evaluated by the work hardening amount defined by “hardness after work−hardness before work”. It means that workability is so good that work hardening amount is large, and 80Hv or more was set as the pass.

(Evaluation of hardness after processing)
Moreover, as evaluation of the hardness after a process, it evaluated by the said post-process hardness measured after the said process test, and made the thing of 250 Hv or more pass.

(Evaluation of surface quality after processing)
Further, the surface quality after processing was evaluated by the presence or absence of SS mark generation after the tensile test. Therefore, JIS Z 2201 No. 5 test piece (25 mm x 50 mm x [plate surface to plate thickness center (plate thickness was adjusted in consideration of the capability of the tensile tester)]) in the direction perpendicular to the rolling direction Based on JIS Z 2241 (1980) (metal material tensile test method), a tensile test was performed up to a strain amount of 15% at a room temperature of 20 ° C. and an initial crosshead speed of 600 mm / min. And the presence or absence of generation | occurrence | production of the SS mark in the surface of the test piece after a tensile test was visually evaluated, after grinding the surface of the test piece with a whetstone. Those in which the SS mark was not generated passed, and those in which the SS mark was generated were rejected.

These measurement results are shown in Table 3 below.

As shown in Table 3, steel no. 1 to 3, 7 to 14, and 25 to 28 all satisfy the requirements of the structure provision of the present invention as a result of manufacturing with the recommended hot rolling conditions using the steel grade that satisfies the requirements of the composition composition of the present invention. Invented steel, the surface properties after processing, the hardness after processing and the amount of work hardening all satisfy the acceptance criteria, exhibiting good cold workability during processing, but with a predetermined surface quality and hardness after processing. It was confirmed that a hot-rolled steel sheet exhibiting thickness (strength) was obtained.

In contrast, Steel No. Reference numerals 4 to 6, 15 to 24, and 29 are comparative steels that do not satisfy at least one of the component composition and the structural requirement defined in the present invention, and are at least one of the surface properties after processing, the hardness after processing, and the work hardening amount. Either does not meet the acceptance criteria.

For example, steel No. No. 4, although the requirements for the component composition are satisfied, the heating temperature before hot rolling is too low outside the recommended range, the amount of solute N is insufficient, and the hardness after processing is inferior.

Steel No. No. 5, although the requirements of the component composition are satisfied, the plate thickness after hot rolling is too large outside the specified range, the bainitic ferrite is insufficient while being coarsened, and the post-working hardness and work hardening amount are inferior. Yes.

Steel No. No. 6, although the requirements of the component composition are satisfied, the final rolling reduction ratio during hot rolling is too small outside the recommended range, and bainitic ferrite is insufficient while coarsening, post-processing hardness and work hardening amount are inferior Yes.

Steel No. No. 15 (steel type j), although the hot rolling conditions are in the recommended range, the N content is too low and the post-working hardness and work hardening amount are inferior.

On the other hand, steel No. No. 16 (steel type k), although the hot rolling conditions are in the recommended range, the N content is too high and not only the cold workability but also the surface properties after processing are inferior.

Steel No. 17 (steel grade l), although the hot rolling conditions are in the recommended range, the C content is too high and does not satisfy the requirement of 10C + N ≦ 3.0, pearlite is excessively formed, not only cold workability, The surface properties after processing are also poor.

Steel No. No. 18 (steel type m) has a hot rolling condition in the recommended range, but the Si content is too high and at least cold workability is poor.

Steel No. Although 19 (steel type n) has a hot rolling condition in the recommended range, the Mn content is too low, and the post-working hardness and work hardening amount are inferior.

On the other hand, steel No. Although 20 (steel type o) has a hot rolling condition in the recommended range, the Mn content is too high and at least cold workability is poor.

Steel No. Although 21 (steel type p) has a hot rolling condition in the recommended range, the P content is too high and at least cold workability is inferior.

Steel No. Although 22 (steel type q) has a hot rolling condition in the recommended range, the S content is too high and at least cold workability is poor.

Steel No. Although 23 (steel type r) has a hot rolling condition in the recommended range, the Al content is too low and at least cold workability is inferior.

On the other hand, steel No. In 24 (steel type s), although the hot rolling conditions other than the final reduction ratio are in the recommended range, the Al content is too high and at least cold workability is poor.

On the other hand, steel No. Although 29 (steel type x) has a hot rolling condition in the recommended range, it does not satisfy the requirement of 10C + N ≦ 3.0, and not only the cold workability but also the surface properties after processing are inferior.

From the above, the applicability of the present invention was confirmed.

Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention.
This application is based on a Japanese patent application filed on September 4, 2013 (Japanese Patent Application No. 2013-183091), the contents of which are incorporated herein by reference.

The hot-rolled steel material of the present invention is useful, for example, for various parts for automobiles (transmission parts such as gears and cases), and can realize weight reduction and high strength.

Claims

The plate thickness is 3-20mm,
Ingredient composition
% By mass (hereinafter the same for chemical components)
C: 0.3% or less (excluding 0%),
Si: 0.5% or less (excluding 0%),
Mn: 0.2 to 1%
P: 0.05% or less (excluding 0%),
S: 0.05% or less (excluding 0%),
Al: 0.01 to 0.1%,
N: 0.008 to 0.025%,
The balance consists of iron and inevitable impurities,
Solid solution N: 0.007% or more, and
The content of C and N satisfies the relationship of 10C + N ≦ 3.0,
Organization
The area ratio for all tissues
Bainitic ferrite: 5% or more
Perlite: less than 20%
The rest: polygonal ferrite,
A hot-rolled steel sheet excellent in cold workability, surface properties and hardness after processing, wherein the average crystal grain size of the bainitic ferrite is in the range of 3 to 50 μm.
The hot rolled steel sheet according to claim 1, wherein the component composition further comprises at least one of the following (a) to (e).
(A) At least one of Cr: 2% or less (not including 0%) and Mo: 2% or less (not including 0%) (b) Ti: 0.2% or less (not including 0%), Nb : 0.2% or less (excluding 0%), V: at least one selected from the group consisting of 0.2% or less (not including 0%) (c) B: 0.005% or less (0% Not included)
(D) At least one selected from the group consisting of Cu: 5% or less (not including 0%), Ni: 5% or less (not including 0%), Co: 5% or less (not including 0%)
(E) Ca: 0.05% or less (not including 0%), REM: 0.05% or less (not including 0%), Mg: 0.02% or less (not including 0%), Li: 0.02% or less (not including 0%), Pb: 0.5% or less (not including 0%), Bi: 0.5% or less (not including 0%) seed