WO2014141919A1

WO2014141919A1 - Hot-rolled steel sheet having excellent drawability and post-processing surface hardness

Info

Publication number: WO2014141919A1
Application number: PCT/JP2014/055296
Authority: WO
Inventors: 梶原　桂
Original assignee: 株式会社神戸製鋼所
Priority date: 2013-03-15
Filing date: 2014-03-03
Publication date: 2014-09-18
Also published as: TWI527909B; JP2014177686A; CN105026596A; US20160002745A1; JP6177551B2; TW201506171A; CN105026596B

Abstract

A hot-rolled steel sheet having a sheet thickness of 2 to 15 mm and having a component composition, by mass, of 0.3% or less of C, 0.5% or less of Si, 0.2 to 1% of Mn, 0.05% or less of P, 0.05% or less of S, 0.01 to 0.1% of Al, and 0.008 to 0.025% of N, with the remainder made up by iron and unavoidable impurities, the solid-solution N being at least 0.007%. In ferrite crystal grains located at the position of depth t/4 (where t is the thickness of the hot-rolled steel sheet), the area ratio of crystal grains within a plane orientation of 10º from the (123) plane is at least 20%, the area ratio of crystal grains within a plane orientation of 10º from the (111) plane is at least 5%, the area ratio of crystal grains within a plane orientation of 10º from the (001) plane is no more than 20%, and the average grain diameter of ferrite crystal grains located at the position of depth t/4 is 3 to 35 μm.

Description

Hot-rolled steel sheet with excellent drawability and surface hardness after processing

The present invention relates to a hot-rolled steel sheet that exhibits good drawing workability during processing and exhibits a predetermined surface 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 life of the cold working die, and the more difficult it is to crack during cold working.

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 technique controls strain aging only by the amount of solute C, and it is difficult to obtain a steel material that has both sufficient cold workability and required hardness and strength after processing. .

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 was found that a steel material for machine structure showing a predetermined surface hardness (strength) was 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, and is a hot forging material, similar to the wire 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. 50% of the content, S: 0.005% or less, N: 0.005% or less, O: 0.004% or less, S, N and O is a total composition containing 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. This hot-rolled steel sheet has excellent dimensional accuracy of a precision stamped surface and surface hardness of the stamped surface after processing. 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.

In general, with regard to texture control for improving the formability of a steel sheet, the deep drawing workability of a thin steel sheet used for a car body outer plate of an automobile is determined by the plastic anisotropy of the material (r value (Rankford value): tensile The greater the ratio between the plate width strain and the plate thickness strain in the test), the higher the workability, and the stronger the {111} plane parallel to the plane orientation in the recrystallized texture, the {100} plane orientation It has been clarified experimentally and theoretically that it is indispensable to improve the deep drawability (see Non-Patent Document 1).

For this reason, various efforts have been made to improve workability by controlling the texture of steel sheets.

For example, in mass%, C: 0.01 to 0.1%, Si: 0.01 to 2%, Mn: 0.05 to 3%, P ≦ 0.1%, S ≦ 0.03%, Al: 0.005 to 2.0%, N ≦ 0.01%, B: 0.0005 to 0.0030, and Ti— (48/12) C— (48/14) N— (48 / 32) A steel plate containing Ti within a range satisfying S ≧ −0.03%, with the balance being Fe and inevitable impurities, and a value obtained by dividing the variation in hardness of the steel plate by its average value is 0.00. A high-strength thin steel sheet having a surface strength of 2 or less and a {110} plane in the rolling direction of 1.7 or less has been proposed (see Patent Document 5).

Further, in mass%, C: more than 0.0005% and less than 0.10%, Si: 1.5% or less, Mn: 0.1% or more and 3.0% or less, P: 0.080% or less, S: 0 0.03% or less, sol. Al: 0.01% or more and 0.50% or less and N: 0.005% or less, and Nb: 0.20% or less and Ti: 0.20% or less The balance is composed of Fe and inevitable impurities, and the steel structure has a volume fraction of 60% or more as a ferrite phase, and is a three-dimensional crystal orientation density function (ODF) {φ1, Φ, φ2}, Φ Is 0 °, φ1 is 0 °, φ2 is 45 °, the intensity of ODF {0 °, 0 °, 45 °} is 3.0 or less, Φ is 35 °, φ1 is 0 °, φ2 A high-strength steel sheet has been proposed in which the strength of ODF {0 °, 35 °, 45 °} when the angle is 45 ° is in the range of 2.5 or more and 4.5 or less. In addition, a high-strength steel sheet having low ductility anisotropy can be provided (see Patent Document 6).

In mass%, C: 0.01 to 0.05%, Si: 0.2% or less, Mn: 0.50% or less, Al: 0.005 to 0.10%, P: 0.05% or less, S: 0.05% or less, N: 0.01% or less, O: 0.01% or less, a composition comprising the balance Fe and inevitable impurities, and a ferrite phase unit having an average crystal grain size of 40 to 60 μm And {118} <110> and {115} <110> have a random intensity ratio of 10 or more, and {111} <110> and {111} <121> each have a random intensity ratio of 1 or less. A hot-rolled steel sheet having a structure has been proposed, and a hot-rolled steel sheet having small in-plane anisotropy after cold rolling-recrystallization annealing can be provided (see Patent Document 7).

As described above, various attempts have been made to improve the formability of steel sheets by texture control. Most of them are related to automotive outer panels, body frame materials, and suspension parts. In transmission parts such as gears and cases targeted by the present invention, the required formability is different from that of automobile body parts, and not only drawing workability and ironing workability are required, but also after processing as described above. It is a component that also requires surface hardness.

Also, in the field of transmission parts, steel bar forging parts (hot forging, cold forging, etc.) are being studied for the production of steel parts with the aim of reducing the weight and cost of parts. Is required.

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 Japanese Unexamined Patent Publication No. 2009-24226 Japanese Unexamined Patent Publication No. 2012-158797 Japanese Unexamined Patent Publication No. 2007-291514

The present invention has been made paying attention to the above circumstances, and an object of the present invention is to provide a hot-rolled steel sheet having a predetermined surface hardness after processing while exhibiting good drawability during processing. .

The invention described in claim 1
The plate thickness is 2-15mm,
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,
Regarding ferrite crystal grains existing at a position of depth t / 4 (t: plate thickness, the same shall apply hereinafter),
The area ratio of ferrite crystal grains whose plate plane orientation is within 10 ° from the (123) plane is 20% or more,
The area ratio of ferrite crystal grains whose plate plane orientation is within 10 ° from the (111) plane is 5% or more,
The area ratio of ferrite crystal grains whose plate plane orientation is within 10 ° from the (001) plane is 20% or less,
A hot-rolled steel sheet having excellent drawing workability and surface hardness after processing, characterized in that the average grain size of ferrite crystal grains existing at the position of the depth t / 4 is 3 to 35 μm.

The invention described in claim 2
Ingredient composition further
Cr: 2% or less (not including 0%) and / or Mo: 2% or less (not including 0%)
The hot-rolled steel sheet according to claim 1, comprising:

The invention according to claim 3
Ingredient composition further
Ti: 0.2% or less (excluding 0%),
Nb: 0.2% or less (excluding 0%),
The hot-rolled steel sheet according to claim 1 or 2, comprising at least one selected from the group consisting of V: 0.2% or less (excluding 0%).

The invention according to claim 4
Ingredient composition further
B: 0.005% or less (excluding 0%)
The hot-rolled steel sheet according to any one of claims 1 to 3, wherein

The invention described in claim 5
Ingredient composition further
Cu: 5% or less (excluding 0%),
Ni: 5% or less (excluding 0%),
The hot-rolled steel sheet according to any one of claims 1 to 4, comprising at least one selected from the group consisting of Co: 5% or less (not including 0%).

The invention described in claim 6
Ingredient composition further
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%),
The hot-rolled steel sheet according to any one of claims 1 to 5, which contains at least one selected from the group consisting of Bi: 0.5% or less (excluding 0%).

According to the present invention, in the structure mainly composed of ferrite having a predetermined average particle diameter, the amount of solute N is ensured and the texture of the hot-rolled steel sheet is controlled to a predetermined structure form, thereby obtaining drawing workability. Even in the parts to be manufactured, the hot-rolled steel sheet that prolongs the life of the mold by increasing the deformability during processing, is less prone to cracking in the steel sheet, and the parts obtained after processing can ensure a predetermined surface hardness Can now be provided.

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 amount of N solid solution is ensured, but the C content is allowed to a higher range, and the texture structure of the steel sheet is changed. The difference is that the ferrite grains are refined while being controlled.

[Thickness of the steel sheet of the present invention: 2 to 15 mm]
First, the steel sheet of the present invention has a thickness of 2 to 15 mm. If the plate thickness is less than 2 mm, rigidity as a structure cannot be secured. On the other hand, if the plate thickness exceeds 15 mm, it is difficult to achieve the texture form defined in the present invention, and the desired drawing workability improvement effect cannot be obtained. A preferred plate thickness is 3 to 14 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 having a great influence on the formation of the structure of the steel sheet, and the structure is a ferrite main structure or a ferrite-pearlite multiphase structure, but the content is to make the ferrite main structure with as little pearlite as possible. It is an element that needs to be restricted. 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 melting of steel becomes difficult, and it becomes difficult to satisfy the strength and surface hardness after deep drawing, so 0.0005% or more, Preferably it 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 surface hardness after deep drawing, so 0.005% or more, more preferably Is 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, when the Mn content is excessive, the deformation resistance becomes excessive and the structure is not uniform due to segregation. Therefore, the content is set to 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 deteriorates, so the content is preferably 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 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 Ln), 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 texture that characterizes the steel sheet of the present invention will be described.

[A texture of the steel sheet of the present invention]
As described above, the steel sheet of the present invention is based on ferrite-based or ferrite-pearlite double phase steel, and is particularly characterized by controlling the plane direction of ferrite crystal grains and the size thereof within a specific range. And

The texture of the steel sheet of the present invention is composed of a multiphase structure of ferrite and pearlite. If excessive pearlite is present, the formability of the steel sheet is deteriorated. Therefore, the pearlite is preferably 10% or less, more preferably 9% or less, and particularly preferably 8% or less in terms of area ratio. The balance is ferrite.

<Ferrite crystal grains present at the position of depth t / 4 (t: plate thickness, the same shall apply hereinafter)
Area ratio of ferrite crystal grains whose plate plane orientation is within 10 ° from the (123) plane: 20% or more,
Area ratio of ferrite crystal grains whose plate plane orientation is within 10 ° from the (111) plane: 5% or more,
Area ratio of ferrite crystal grains whose plate plane orientation is within 10 ° from the (001) plane: 20% or less>
The formation of the texture differs depending on the processing method even if the crystal system is the same, and in the case of a rolled material, it is expressed by the rolling surface and the rolling direction. In the present invention, it is expressed by a rolled surface (000). In addition, (circle) has shown the integer. The expression of each of these directions is described in, for example, edited by Shinichi Nagashima: “Group Texture” (published by Maruzen Co., Ltd.).

In the present invention, the ferrite crystal grains present at the position of the depth t / 4 have an area ratio of ferrite crystal grains having a plate plane orientation within 10 ° from the (123) plane of 20% or more, and from the (111) plane to 10%. By controlling the area ratio of ferrite crystal grains within 5 ° to 5% or more and the area ratio of ferrite crystal grains within 10 ° from the (001) plane to 20% or less, the drawability of the hot rolled steel sheet is improved. be able to.

Conventionally, with regard to the plate orientation of ferrite crystal grains, it has been known that the (111) plane orientation parallel to the plate surface is strongly developed while the (001) plane orientation is effective for improving deep drawability. ing. Although the sheet surface orientation can be controlled in the cold rolling process and the annealing process, the hot rolling process has a thickness of 2 to 15 mm and the thickness of the steel sheet is 2 to 15 mm. With a steel plate, it is difficult to control the orientation of the plate surface.

Therefore, in the present invention, by newly introducing ferrite crystal grains having a (123) plane, it is possible to control the texture in the hot rolled sheet and realize improvement in formability.

As described above, the ferrite crystal grains having the (123) plane as the plate surface orientation have the effect of improving the formability and deep drawability. 20% or more is necessary. Preferably it is 22% or more, more preferably 24% or more, particularly preferably 26% or more.

The (111) plane also has an effect of improving moldability and deep drawability, and in order to effectively exhibit such an effect, the area ratio is 5% or more, preferably 6% or more, more preferably 8% or more is necessary.

(001) plane causes in-plane anisotropy due to molding and deteriorates moldability. Therefore, the area ratio is limited to 20% or less, preferably 18% or less, and more preferably 15% or less.

In addition, although the structure | tissue form of a hot-rolled steel plate has a structure distribution in the plate | board thickness direction, the structure | tissue form was prescribed | regulated by making the depth position of 1/4 of plate | board thickness into a representative position. In addition, it is considered that ferrite crystal grains whose plate plane orientation is within 10 ° from each of the ideal plane orientations ((123) plane, (111) plane, (001) plane) have almost the same action. Each area ratio of ferrite crystal grains having a plate surface orientation is defined.

<Average diameter of ferrite crystal grains existing at a position of depth t / 4: 3 to 35 μm>
The average grain size of the ferrite crystal grains constituting the ferrite structure is 3 to 35 μm in order to improve the workability (drawing workability, bending workability, press workability) of the steel sheet and satisfy the surface properties after working. It must be in range. If the ferrite crystal grains become too fine, the deformation resistance becomes too high, so the average grain size is 3 μm or more, preferably 4 μm or more, and more preferably 5 μm or more. On the other hand, if the ferrite crystal grains become too coarse, the toughness, fatigue characteristics, etc. deteriorate, and even if the crystal orientation is controlled, the press formability such as bending workability and overhang is remarkably lowered, and cracks during molding Since defects such as rough skin are likely to occur, the average particle size is 35 μm or less, preferably 30 μm or less, and more preferably 28 μm or less. Similarly to the above, the hot rolled steel sheet has a ferrite crystal grain size distribution in the sheet thickness direction, but the average grain diameter of the ferrite crystal grain is defined with a depth position of 1/4 of the sheet thickness as a representative position.

[Measurement method of area ratio of each phase]
About the area ratio of each said phase, each test steel plate carries out a nital corrosion, and 5 field-of-views are image | photographed with a scanning electron microscope (SEM; magnification 1000 times), and each ratio of a ferrite and pearlite can be calculated | required with a point method.

[Measurement method of plate orientation of ferrite crystal grains]
The plate surface orientation of the ferrite crystal grains is measured and analyzed by SEM-EBSP [Electron Back Scattering Pattern and EBSD (Electron Back Scattering Diffraction). As the SEM device, for example, SEM (JEOLJSM5410) manufactured by JEOL Ltd. is used, and for example, EBSP: manufactured by TSL (OIM) is used as the EBSP measurement analysis system. Although it depends on the size of the crystal grains, the measurement area of the sample is 300 to 1000 μm × 300 to 1000 μm, and the measurement step interval is, for example, 1 to 3 μm or less. From the crystal orientation of each ferrite crystal grain thus identified, the total area is obtained by summing the orientations within 10 ° from each ideal plane orientation, and divided by the area of the measurement region. The area ratio for each direction was determined.

[Measurement method of average grain size of ferrite crystal grains]
The average grain size of the ferrite crystal grains is determined by measuring the maximum diameter of each ferrite crystal grain observed in a predetermined measurement region using the SEM-EBSP and measurement conditions thereof, and calculating the average value of the average grain diameters. Obtained as the diameter.

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 870 ° C. or higher. If the finish rolling temperature is too low, ferrite transformation occurs at a high temperature, the precipitated carbides in the ferrite become coarse, and the fatigue strength deteriorates. Therefore, a certain 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 ferrite grain size to some extent. The upper limit of the finish rolling temperature is set to 1000 ° C. because it is difficult to secure the temperature.

[Hot rolling pass schedule]
The thickness of the hot-rolled steel sheet of the present invention is 2 to 15 mm. In order to refine the ferrite crystal grains and control the average grain size within a predetermined grain size range, not only the control of the rolling temperature described above. The final reduction ratio of tandem rolling of finish rolling needs to be 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.

[Rolling speed of hot rolling]
In order to obtain a texture of ferrite crystal grains having the plate surface orientation as described above and to control the texture in the thickness direction to be as uniform as possible, it is necessary to control the rolling speed of finish rolling. For this reason, the rolling speed of the final pass is controlled to 300 to 700 m / min. If the rolling speed is too high or too low, it is difficult to obtain a desired plate surface orientation, and the texture in the plate thickness direction tends to be uneven, which is not preferable. In addition, when the rolling speed is low, the productivity also deteriorates. Preferably, it is 350 to 650 m / min, more preferably 400 to 600 m / mim.

[Rapid cooling after hot rolling]
After completion of the finish rolling, quenching is performed at a cooling rate (first quenching rate) of 20 ° C./s or more within 5 s, and quenching is stopped at a temperature of 580 ° C. or more and less than 670 ° C. (quenching stop temperature). This is to obtain a ferrite-based structure, that is, a ferrite-pearlite double phase structure in which the pearlite fraction is within an allowable range. 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 580 ° C., the pearlite transformation or the bainite transformation is promoted. It becomes difficult to obtain pearlite steel, it cannot be controlled to a desired texture, and drawability and bending workability deteriorate. On the other hand, when the quenching stop temperature is 670 ° C. or higher, the precipitated carbide in the ferrite is coarsened, and the drawing workability and bending workability are similarly deteriorated. The quenching stop temperature is preferably 600 to 650 ° C, more preferably 610 to 640 ° 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. Thus, the precipitated carbide in the ferrite is appropriately refined while sufficiently progressing the formation of the ferrite. When the cooling rate exceeds 10 ° C./s or the slow cooling time is less than 5 s, the amount of ferrite formed is insufficient and the desired texture cannot be controlled, and the drawing workability and bending workability deteriorate. To do. 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 of 20 ° C./s or higher (second rapid cooling rate), and wound up at a temperature exceeding 300 ° C. and not exceeding 450 ° C. This is because, by making the structure mainly composed of ferrite, it is desirable to secure the drawing workability and the bending workability by forming the texture. When the cooling rate (second quenching rate) is less than 20 ° C./s, or when the coiling temperature exceeds 450 ° C., a lot of pearlite is formed, whereas when it is less than 300 ° C., martensite and retained austenite are formed, and drawing workability. , Bending workability deteriorates.

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 thus obtained, the amount of solute N, the area ratio of each phase of the structure in the steel sheet, and the plate surface orientation and average grain size of the ferrite crystal grains are the above [for carrying out the invention] It was determined by each measuring method described in the section of [Form].

Further, the drawing workability of the hot-rolled steel sheet is a steel sheet having a thickness of about 4 to 10 mm, and was evaluated by a cylindrical forming test. The punch size was 100 mm, punch shoulder R8 mm, die diameter 103 mm, and die shoulder R8 mm. The maximum blank diameter at which the cylinder can be squeezed without causing breakage by one squeezing is defined as Dmax, with the ratio (D / d) of the blank diameter (D) to the cylinder diameter (d). At this time, Dmax / d was defined as “limit drawing ratio” (LDR (Limiting Drawing Ratio)), and this was used as an evaluation index. An LDR of 2 to 2.1 was accepted.

Also, the molded specimen was taken out and the outside of the cylindrical part and the flange part was visually observed. As a result of visual observation, x is observed when cracks are observed, no cracks are generated, but Δ is observed when visible cracks are observed, but fine irregularities (wrinkles) are observed, but no cracks are generated. The case was marked with ◯, and the case without wrinkles was marked with ◎. And the thing of (double-circle) or (circle) was set as the pass. In addition, “crack” and “crack” were distinguished by defining a crack having a maximum width of 1 mm or more as “crack” and a crack having a maximum width of less than 1 mm as “crack”.

Also, the hardness of the surface portion of the flange portion after the drawing test was measured, and the surface hardness after processing was evaluated. Using a Vickers hardness tester, load: 1000 g, measurement position: D / 4 position center of test piece cross section (D: part diameter) and number of measurements: 5 times Vickers hardness of each processed test piece The thickness (Hv) was measured. And the thing more than 250Hv was set as the pass.

These measurement results are shown in Table 3 below. In addition, the notation of “(xxx) plane (%)” in the column of “ferrite crystal grain” in the table indicates the area ratio of ferrite crystal grains whose plate plane orientation is within 10 ° from the (xxx) plane ( Unit:%).

As shown in Table 3, steel no. 1, 2, 7 to 14, 25 to 31 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 compositional composition of the present invention. Inventive steel, drawing workability and post-working surface hardness all meet the acceptance criteria, while exhibiting good drawing workability during working, and exhibiting a predetermined surface hardness (strength) after working It was confirmed that a rolled steel sheet was obtained.

In contrast, Steel No. 3 to 6 and 15 to 24 are comparative steels that do not satisfy at least one of the component composition and the structure requirements specified in the present invention, and at least one of drawing workability and post-working surface hardness meets the acceptance criteria. not filled.

For example, steel No. 3, although the requirements of the component composition are satisfied, the heating temperature before hot rolling, the finish rolling temperature during hot rolling, and the quenching stop temperature after hot rolling are too low outside the recommended range, and the amount of solute N is too low. In addition to the shortage of ferrite crystal grains with the (123) plane orientation, ferrite crystal grains with the (001) plane orientation become excessive, and both drawability and surface hardness after processing are inferior.

Steel No. 4, while the requirements of the component composition are satisfied, the plate thickness after hot rolling is too large outside the specified range, while the ferrite grains of (123) plane orientation and (111) plane orientation are both insufficient ( The (001) plane orientation ferrite crystal grains become excessive, the ferrite crystal grains become coarse, and at least the drawability is inferior.

Steel No. 5, although the requirements of the component composition are satisfied, the rolling speed of the final pass at the time of hot rolling is too large outside the recommended range, while (123) face orientation ferrite crystal grains are insufficient, while (001) face orientation The ferrite crystal grains become excessive and the drawing processability is inferior.

Steel No. No. 6 satisfies the requirements of the component composition, but the final rolling reduction during hot rolling is too small outside the recommended range, the ferrite crystal grains are coarsened, and at least the drawability is inferior.

Steel No. Although 15 (steel type j) has a hot rolling condition in the recommended range, since the N content is too low, the amount of solute N is insufficient and the surface hardness after processing is inferior.

On the other hand, steel No. In No. 16 (steel type k), the hot rolling conditions are in the recommended range, but the N content is too high and at least the drawability is inferior.

Steel No. No. 17 (steel type l) has a hot rolling condition in the recommended range, but the C content is too high, the amount of solute N is insufficient, and the surface hardness after processing is inferior.

Steel No. No. 18 (steel type m), although the hot rolling conditions are in the recommended range, the Si content is too high, and (123) plane orientation ferrite crystal grains are insufficient, while (001) plane orientation ferrite crystal grains are excessive. Therefore, at least the drawability is inferior.

Steel No. 19 (steel type n), although the hot rolling conditions are in the recommended range, the Mn content is too low, the ferrite grains with the (001) orientation are excessive, and the drawability and surface hardness after processing are inferior. Yes.

On the other hand, steel No. 20 (steel type o), although the hot rolling conditions are in the recommended range, the Mn content is too high, and the ferrite crystal grains with (123) orientation are insufficient, while the ferrite grains with (001) orientation are excessive. Therefore, at least the drawability is inferior.

Steel No. 21 (steel type p), although the hot rolling conditions are in the recommended range, the P content is too high, and (123) plane orientation ferrite crystal grains are insufficient, while (001) plane orientation ferrite crystal grains are excessive. Therefore, at least the drawability 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 ferrite grains of (123) plane orientation and (111) plane orientation are insufficient, while (001) plane orientation Ferrite crystal grains become excessive, and at least drawing workability is inferior.

Steel No. 23 (steel type r), although the hot rolling conditions are in the recommended range, the Al content is too low, and (123) plane orientation ferrite crystal grains are insufficient, while (001) plane orientation ferrite crystal grains are excessive. Therefore, at least the drawability is inferior.

On the other hand, steel No. 24 (steel type s), although the hot rolling conditions are in the recommended range, the Al content is too high, and (123) plane orientation ferrite crystal grains are insufficient, while (001) plane orientation ferrite crystal grains are excessive. Therefore, at least the drawability is 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 March 15, 2013 (Japanese Patent Application No. 2013-053564), the contents of which are incorporated herein by reference.

The hot-rolled steel sheet of the present invention is used for transmission parts such as automobile gears, cases, and the like, and it is possible to reduce the weight and strength of these parts.

Claims

The plate thickness is 2-15mm,
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,
Regarding ferrite crystal grains existing at a position of depth t / 4 (t: plate thickness, the same shall apply hereinafter),
The area ratio of ferrite crystal grains whose plate plane orientation is within 10 ° from the (123) plane is 20% or more,
The area ratio of ferrite crystal grains whose plate plane orientation is within 10 ° from the (111) plane is 5% or more,
The area ratio of ferrite crystal grains whose plate plane orientation is within 10 ° from the (001) plane is 20% or less,
A hot-rolled steel sheet excellent in drawing workability and surface hardness after processing, characterized in that the average grain size of ferrite crystal grains present at the position of the depth t / 4 is 3 to 35 μm.
The hot rolled steel sheet according to claim 1, wherein the component composition further comprises at least one of (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 (not including 0%) and V: 0.2% or less (not including 0%)
(C) B: 0.005% or less (excluding 0%)
(D) At least one of Cu: 5% or less (not including 0%), Ni: 5% or less (not including 0%), and 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: At least one of 0.02% or less (excluding 0%), Pb: 0.5% or less (not including 0%), and Bi: 0.5% or less (not including 0%)