US20150329932A1

US20150329932A1 - Hot-rolled steel sheet exhibiting excellent cold formability and excellent surface hardness after forming

Info

Publication number: US20150329932A1
Application number: US14/652,235
Authority: US
Inventors: Katsura Kajihara
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2013-01-10
Filing date: 2014-01-10
Publication date: 2015-11-19
Also published as: CN104903484B; CN104903484A; JP6058439B2; TWI513830B; TW201443245A; JP2014148739A; WO2014109401A1

Abstract

Disclosed is a hot-rolled steel sheet that has a thickness of 3 to 20 mm and includes, in a chemical composition in mass percent, C of 0.3% or less (excluding 0%), Si of 0.5% or less (excluding 0%), Mn of 0.2% to 1%, P of 0.05% or less (excluding 0%), S of 0.05% or less (excluding 0%), Al of 0.01% to 0.1%, and N of 0.008% to 0.025%, with the remainder consisting of iron and inevitable impurities. A solute nitrogen content is 0.007% or more, and the carbon and nitrogen contents meet a condition as specified by 10C+N≦3.0. The microstructure of the steel sheet includes pearlite of less than 20% in area percentage, with the remainder approximately consisting of ferrite. The average grain size of the ferrite is 3 to 35 μm. The steel sheet has good cold formability during forming and still has predetermined surface hardness after forming.

Description

TECHNICAL FIELD

The present invention relates to a hot-rolled steel sheet that has good cold formability during forming and still has predetermined surface hardness after forming.

BACKGROUND ART

Recently, better fuel efficiency is required in automobiles from the viewpoint of environmental protection. To meet the requirement, steels for use in automobile parts such as gears and other gearbox unit parts and casings more and more require lighter weights, i.e., higher strength. To meet the requirement of lighter weights and higher strength, hot-forged steels prepared from steel bars by hot forging have been generally used. Instead of the hot forged gears and other parts, demands are increasingly made to provide these parts by cold forging so as to reduce CO₂emission in parts production processes.
Advantageously, the cold forming (cold forging) offers higher productivity and provides both good dimensional accuracy and good steel yield as compared with hot forming and warm forming. Disadvantageously, however, the cold forming, when employed to produce parts, has to essentially use steels having high strength, i.e., high deformation resistance so as to allow the cold-worked parts to surely have strength at predetermined levels or higher. Unfortunately, steels with increasing deformation resistance may more readily invite shorter lives of cold-forming tools and more readily cause fracture/cracking upon cold forming.
To prevent this, some conventional techniques produce high-strength parts surely having predetermined strength (hardness) by cold-forging a steel into a predetermined shape, and subjecting the cold-forged steel to a heat treatment such as quenching and tempering. However, the parts inevitably change their dimensions in the heat treatment after cold forging and thereby require secondary correction by machining such as cutting. Under these circumstances, demands have been made to provide a solution that can omit the heat treatment and the subsequent forming.
As a possible solution to the problems, for example, Patent Literature (PTL) 1 discloses that a wire rod/steel bar for cold forging having excellent strain aging hardening properties is obtained by preparing a low-carbon steel, restraining the progress of natural aging of the steel using solute carbon, and allowing the steel to surely undergo age hardening by strain aging hardening at a predetermined level.
This technique, however, controls the strain aging hardening by the solute carbon content alone and hardly gives a steel that has both sufficient cold formability and required level of hardness/strength after forming.
Under the circumstances, the present applicant made various investigations while focusing on the effect of solute carbon and solute nitrogen in a steel on deformation resistance and static strain aging hardening. As a result, the present applicant found that appropriate control of the amounts of these solute elements gives a mechanical-structure-use steel that exhibits good cold formability during forming and still has predetermined surface hardness (strength) after cold forming (cold forging). The present applicant has already filed a patent application based on these findings (see PTL 2).
The steel achieves both good cold formability and higher hardness (higher strength) after forming. Disadvantageously, however, the steel is a hot-forged steel as with the wire rod/steel bar disclosed in PTL 1 and suffers from high production cost. To achieve still lower production cost, attempts have been made to produce automobile parts by cold forming using hot-rolled steel sheets instead of the conventional hot-forged steels.
Typically, PTL 3 proposes a technique, according to which a hot-rolled steel sheet for nitriding can have high surface hardness and a sufficient hardening depth after nitriding.
Disadvantageously, however, the technique requires nitriding as an extra process after cold forming and fails to offer sufficiently low cost.
PTL 4 proposes a hot-rolled steel sheet that has a chemical composition containing C in a content of 0.10% or less, Si in a content less than 0.01%, Mn in a content of 1.5% or less, Al in a content of 020% or less, Ti and Nb in a content as specified by (Ti+Nb)/2 of 0.05% to 0.50%, Sin a content of 0.005% or less, N in a content of 0.005% or less, O in a content of 0.004% or less, so that the total content of S, N, and O be 0.0100% or less. The hot-rolled steel sheet has a microstructure containing 95% or more of ferrite as approximately a ferrite single-phase. The literature mentions that the hot-rolled steel sheet has excellent dimensional accuracy in a finely blanked surface, has extremely high surface hardness of the blanked surface after forming, and still offers excellent resistance to red-scale defects.
The hot-rolled steel sheet, however, is designed to treat nitrogen as a harmful element and to control the nitrogen content to an extremely low content and absolutely differs in technical idea from the hot-rolled steel sheet according to the present invention in which nitrogen is positively utilized.

CITATION LIST

Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication (JP-A) No. Hei10(1998)-306345
PTL 2: JP-A No. 2009-228125
PTL 3: JP-A No. 2007-162138
PTL 4: JP-A No. 2004-137607

SUMMARY OF INVENTION

Technical Problem

The present invention has been made while focusing these circumstances, and it is an object of the present invention to provide a hot-rolled steel sheet that has good cold formability during forming and still has predetermined surface hardness after forming.

Solution to Problem

The present invention provides, in one aspect, a hot-rolled steel sheet having excellent cold formability and satisfactory surface hardness after forming. The hot-rolled steel sheet has a thickness of 3 to 20 mm and contains, in a chemical composition in mass percent (hereinafter the same for chemical composition), C in a content of 0.3% or less (excluding 0%), Si in a content of 0.5% or less (excluding 0%), Mn in a content of 0.2% to 1%, P in a content of 0.05% or less (excluding 0%), S in a content of 0.05% or less (excluding 0%), Al in a content of 0.01% to 0.1%, and N in a content of 0.008% to 0.025%, with the remainder approximately consisting of iron and inevitable impurities. In the hot-rolled steel sheet, the content of solute nitrogen is 0.007% or more, and the contents of carbon and nitrogen meet a condition as specified by the expression: 10C+N≦3.0. The microstructure of the hot-rolled steel sheet indudes pearlite in a content of less than 20% in area percentage based on the total microstructure, with the remainder approximately consisting of ferrite. The ferrite has an average grain size of 3 to 35 μm.
In an embodiment, the hot-rolled steel sheet according to the aspect may further contain, in the chemical composition, Cr in a content of 2% or less (excluding 0%) and/or Mo in a content of 2% or less (excluding 0%).
In another embodiment, the hot-rolled steel sheet according to the aspect may further contain, in the chemical composition, at least one element selected from the group consisting of Ti in a content of 0.2% or less (excluding 0%), Nb in a content of 0.2% or less (excluding 0%), and V in a content of 0.2% or less (excluding 0%).
In yet another embodiment, the hot-rolled steel sheet according to the aspect may further contain, in the chemical composition, B in a content of 0.005% or less (excluding 0%).
In yet another embodiment, the hot-rolled steel sheet according to the aspect may further contain, in the chemical composition, at least one element selected from the group consisting of Cu in a content of 5% or less (excluding 0%), Ni in a content of 5% or less (excluding 0%), and Co in a content of 5% or less (excluding 0%).
In still another embodiment, the hot-rolled steel sheet according to the aspect may further contain, in the chemical composition, at least one element selected from the group consisting of Ca in a content of 0.05% or less (excluding 0%), at least one rare-earth element (REM) in a total content of 0.05% or less (excluding 0%), Mg in a content of 0.02% or less (excluding 0%), Li in a content of 0.02% or less (excluding 0%), Pb in a content of 0.5% or less (excluding 0%), and Bi in a content of 0.5% or less (excluding 0%).

Advantageous Effects of Invention

According to the present invention, a microstructure mainly containing ferrite having a predetermined average grain size is controlled so that the microstructure contains solute nitrogen in a certain amount, and the carbon content and the nitrogen content meet a predetermined condition. This can provide a hot-rolled steel sheet that has lower deformation resistance during cold forming, thereby contributes to longer lives of tools, still resists fracture/cracking, and gives, after forming, a part surely having predetermined surface hardness.

DESCRIPTION OF EMBODIMENTS

The hot-rolled steel sheet according to the present invention will be illustrated in detail below. The hot-rolled steel sheet according to the present invention is hereinafter also referred to as “steel sheet according to the present invention” or simply referred to as “steel sheet”. The steel sheet according to the present invention has a commonality with the hot-forged steel disclosed in PTL 2 in that solute nitrogen is contained in a certain amount and that the carbon content and the nitrogen content are controlled so as to meet a predetermined condition. The steel sheet according to the present invention, however, differs from the hot-forged steel in that the upper limit of the carbon content is relatively high, the steel sheet is controlled to include a ferrite-pearlite dual-phase microstructure as the microstructure, and ferrite grains are refined.
The steel sheet according to the present invention has a thickness of 3 to 20 mm.
First of all, the steel sheet according to the present invention is directed to one having a thickness of 3 to 20 mm. The steel sheet, if having a thickness of less than 3 mm, may fail to surely have rigidity as a structure. In contrast, the steel sheet, if having a thickness greater than 20 mm, may hardly have the microstructure in the form as specified in the present invention and hardly have desired effects. The steel sheet preferably has a thickness of 4 to 19 mm.
Next, the chemical composition constituting the steel sheet according to the present invention will be described. All chemical elements hereinafter are indicated in mass percent.
Chemical Composition of Steel Sheet According to Present Invention
C in a Content of 0.3% or Less (Excluding 0%)
Carbon (C) significantly affects the formation of steel sheet microstructure, and the content thereof may be controlled so as to form a microstructure that is a ferrite-pearlite dual-phase microstructure, but mainly contains ferrite and contains pearlite in a minimized amount. The steel sheet, if containing carbon in excess, may have a higher pearlite fraction in the microstructure and might have excessively high deformation resistance due to pearlite's work hardening. To prevent this, the carbon content in the steel sheet may be controlled to 0.3 percent by mass or less, preferably 0.25% or less, more preferably 0.2% or less, and particularly preferably 0.15% or less. However, the steel sheet, if having an excessively low carbon content, may hardly undergo deoxidation during steel ingot making. To prevent this, the carbon content may be controlled to preferably 0.0005% or more, more preferably 0.0008% or more, and particularly preferably 0.001% or more.
Si in a Content of 0.5% or Less (Excluding 0%)
Silicon (Si) dissolves in the steel, causes the steel sheet to have higher deformation resistance, and has to be minimized. To reduce deformation resistance, the Si content in the steel sheet may be controlled to 0.5% or less, preferably 0.45% or less, more preferably 0.4% or less, and particularly preferably 0.3% or less. However, the steel sheet, if having an extremely low Si content, may hardly undergo deoxidation during steel ingot making. To prevent this, the Si content may be controlled to preferably 0.005% or more, more preferably 0.008% or more, and particularly preferably 0.01% or more.
Mn in a Content of 0.2% to 1%
Manganese (Mn) effectively deoxidizes and desulfurizes in the steel making process. Assuming that the steel has a higher nitrogen content, in this case, the steel sheet may be susceptible to fracture/cracking by dynamic strain aging hardening with the heat generation by mechanical forming. Manganese, however, effectively contributes to better formability in this process and restrains fracture/cracking. To have these activities effectively, the Mn content in the steel sheet may be controlled to 0.2% or more, preferably 0.22% or more, and more preferably 0.25% or more. However, the steel sheet, if containing Mn in excess, may have excessively high deformation resistance and may suffer from a heterogeneous microstructure due to segregation. To prevent this, the Mn content may be controlled to 1% or less, preferably 0.98% or less, and more preferably 0.95 percent by mass or less.
P in a Content of 0.05% or Less (Excluding 0%)
Phosphorus (P) is an impurity element and is inevitably contained in the steel. Phosphorus, if contained in ferrite, segregates at ferrite grain boundaries to impair cold formability. This element also causes ferrite to undergo solute strengthening and cause the steel sheet to have higher deformation resistance. To prevent this and to offer good cold formability, the phosphorus content is preferably minimized. However, extreme minimization of the phosphorus content may bring about an increase in steel making cost. To prevent this in consideration of process capability, the phosphorus content may be controlled to 0.05% or less, and preferably 0.03% or less.
S in a Content of 0.05% or Less (Excluding 0%)
Sulfur (S) is an inevitable impurity as with phosphorus, precipitates as a film of FeS at grain boundaries, and impairs formability. This element also causes hot brittleness. To prevent this and to provide better deformability, the sulfur content herein may be controlled to 0.05% or less, and preferably 0.03% or less. It is industrially difficult, however, to control the sulfur content to zero (0). However, sulfur effectively allows the steel sheet to have better machinability. For better machinability, it is recommended for the steel sheet to contain sulfur in a content of preferably 0.002% or more, and more preferably 0.006% or more.
Al in a Content of 0.01% to 0.1%
Aluminum (Al) effectively contributes to deoxidation in the steel making process. To have the deoxidation effect, the steel sheet may have an Al content of 0.01% or more, preferably 0.015% or more, and more preferably 0.02% or more. However, the steel sheet, if having an excessively high Al content, may have lower toughness and be susceptible to fracture/cracking. To prevent this, the Al content may be controlled to 0.1% or less, preferably 0.09% or less, and more preferably 0.08 percent by mass or less.
N in a Content of 0.008% to 0.025%
Nitrogen (N) causes static strain aging hardening after forming and thereby allows the steel sheet to have predetermined strength, thus being important. For this reason, the nitrogen content in the steel sheet may be controlled to 0.008% or more, and preferably 0.0085% or more, and more preferably 0.009% or more. However, the steel sheet, if having an excessively high nitrogen content, may be significantly affected not only by static strain aging hardening, but also by dynamic strain aging hardening during forming to have higher deformation resistance, thus being unsuitable. To prevent this, the nitrogen content may be controlled to 0.025% or less, preferably 0.023 percent by mass or less, and more preferably 0.02% or less.
Solute Nitrogen in a Content of 0.007% or More
The steel sheet includes solute nitrogen in a predetermined amount. This accelerates static strain aging hardening with less increase in deformation resistance. The amount of solute nitrogen is hereinafter also referred to as “solute nitrogen content”. The steel sheet may have a solute nitrogen content of 0.007% or more so as to surely have required strength after cold forming. However, the steel sheet, if having an excessively high solute nitrogen content, may have inferior cold formability. To prevent this, the solute nitrogen content is preferably controlled to 0.03% or less. As the total nitrogen content in the steel sheet is 0.025% or less, the solute nitrogen content does not approximately exceed 0.025%.
As used herein the term “solute nitrogen content” refers to the amount as determined by subtracting the total amount of nitrogen compounds from the total nitrogen content in the steel sheet. The determination is performed based on Japanese Industrial Standard (JIS) G 1228. Practical methods for measuring the solute nitrogen content are exemplified as follows:
(a) Inert Gas Fusion-Thermal Conductivity Analysis (Total Nitrogen Content Measurement)
A specimen is cut out from a test sample, placed in a crucible, and fused in an inert gas stream to extract nitrogen. The extract is transferred to a thermal conductivity cell to measure a change in thermal conductivity to thereby determine the total nitrogen content.
(b) Ammonia Separation by Distillation-Indophenol Blue Absorptiometry (Total Nitrogen Compound Amount Measurement)
A specimen is cut out from the test sample, dissolved in a 10% AA electrolytic solution, subjected to constant current electrolysis to measure the total amount of nitrogen compounds in the steel (steel sheet). The 10% AA electrolytic solution to be used is a non-aqueous electrolytic solution that contains 10% of acetone and10% of tetramethylammonium chloride with the remainder being methanol and does not form a passive film on the steel surface.
About 0.5 g of the specimen sampled from the test sample is dissolved in the 10% AA electrolytic solution, and formed undissolved residue (nitrogen compounds) is filtered through a polycarbonate filter having a pore size of 0.1 μm. The undissolved residue is heated in sulfuric acid, potassium sulfate, and pure copper chips and is decomposed, and the decomposed product is combined with the filtrate. The resulting mixture (solution) is treated with sodium hydroxide to be basic, subjected to steam distillation, and distilled ammonia is absorbed by diluted sulfuric acid. This is further combined with phenol, sodium hypochlorite, and sodium pentacyanonitrosylferrate(III) to form a blue complex, and the absorbance of the blue complex is measured using an absorptiometer to determine the total amount of nitrogen compounds.
The total amount of nitrogen compounds determined by the method (b) is subtracted from the total nitrogen content determined by the method (a) to give the solute nitrogen content.
The carbon and nitrogen contents meet the condition as specified by the expression: 10C+N≦3.0
In the steel sheet according to the present invention, the solute carbon contributes to significantly better deformation resistance, but less contributes to static strain aging hardening. In contrast, the solute nitrogen less contributes to higher deformation resistance, but can accelerate static strain aging hardening and can effectively contribute to higher hardness after forming. Based on this, the steel sheet according to the present invention essentially has the carbon content and the nitrogen content meeting the condition as specified by the expression: 10C+N≦3.0. The condition between the two elements is preferably 0.009≦10C+N≦2.8, more preferably 0.01≦10C+N≦2.5, and particularly preferably 0.01≦10C+N≦2.0. The condition is specified so as to increase the hardness after forming with less causing the increase of deformation resistance during forming. The steel sheet may have a carbon content and a solute carbon content at certain levels, for the grain refinement in the hot-rolled steel sheet and for the formability of the steel sheet at certain level. However, the steel sheet, if having carbon and nitrogen contents not meeting the condition (if 10C+N is greater than 3.0), may have excessively high deformation resistance due to excessive content(s) of carbon and/or nitrogen. In the inequality, the coefficient of the carbon content is set 10 times the coefficient of the nitrogen content. This is set in consideration that the solute carbon, even contained in the same content with the solute nitrogen, increases the strength and deformation resistance of the hot-rolled steel sheet according to the present invention to a degree greater by an order of magnitude (10 times) as compared with the solute nitrogen.
The steel sheet according to the present invention basically contains the chemical composition (elements) with the remainder approximately consisting of iron and inevitable impurities. The steel sheet may further contain any of following allowable elements within ranges not adversely affecting the operation of the present invention.
Cr in a Content of 2% or Less (Excluding 0%) and/or Mo in a Content of 2% or Less (Excluding 0%)
Chromium (Cr) increases grain boundary strength to effectively allow the steel to have better deformability. To have such activities effectively, the steel sheet preferably contain Cr in a content of 0.2%. However, the steel sheet, if containing Cr in excess, may have higher deformation resistance and deteriorated cold formability. To prevent this, the Cr content may be controlled to preferably 2% or less, more preferably 1.5% or less, and particularly preferably 1% or less.
Molybdenum (Mo) effectively allows the steel sheet to have higher hardness after forming and better deformability. To have such activities effectively, the steel sheet may contain Mo in a content of preferably 0.04% or more, and more preferably 0.08% or more. However, the steel sheet, if containing Mo in excess, may have inferior cold formability. To prevent this, the Mo content may be controlled to preferably 2% or less, more preferably 1.5% or less, and particularly preferably 1% or less.
At Least One Element Selected from the Group Consisting of Ti in a Content of 0.2% or Less (Excluding 0%), Nb in a Content of 0.2% or Less (Excluding 0%), and V in a Content of 0.2% or Less (Excluding 0%)
These elements have a high affinity for nitrogen, are coexistent with nitrogen to form nitrogen compounds, and contribute to grain refinement of the steel. These elements also allow the formed product (steel sheet) after cold forming to have better toughness and better resistance to fracture/cracking. Each of the elements, however, fails to offer further better properties when contained in a content greater than the upper limit. To prevent this, the contents of the elements may each be controlled to preferably 0.2% or less, more preferably 0.001% to 0.15%, and particularly preferably 0.002% to 0.1%.
B in a Content of 0.005% or Less (Excluding 0%)
Boron (B) acts similarly to Ti, Nb, and V mentioned above. Specifically, boron has a high affinity for nitrogen, is coexistent with nitrogen to form nitrogen compounds, and contributes to grain refinement in the steel. This element also allows the formed product (steel sheet) after cold forming to have better toughness and better resistance to fracture/cracking. The steel sheet according to the present invention, when containing boron, can have a required solute nitrogen content and have higher strength after cold forming. The boron content may therefore be preferably 0.005% or less, more preferably 0.0001% to 0.0035%, and particularly preferably 0.0002% to 0.002%.
At Least One Element Selected from the Group Consisting of Cu in a Content of 5% or Less (Excluding 0%), Ni in a Content of 5% or Less (Excluding 0%), and Co in a Content of 5% or Less (Excluding 0%)
These elements each effectively allow the steel to undergo strain aging hardening and to be hardened and effectively allow the steel sheet to have higher strength after forming. To have such activities effectively, each of these elements may be contained in a content of preferably 0.1% or more, and more preferably 0.3% or more. However, these elements, if contained in excess, may have saturated effects of allowing the steel to undergo strain aging hardening and to be hardened and allowing the steel sheet to have higher strength after forming and may accelerate fracture/cracking. To prevent this, the contents of these elements may each be controlled to preferably 5% or less, more preferably 4% or less, and particularly preferably 3% or less.
At Least One Element Selected from the Group Consisting of Ca in a Content of 0.05% or Less (Excluding 0%), at Least One Rare-Earth Element (REM) in a Total Content of 0.05% or Less (Excluding 0%), Mg in a Content of 0.02% or Less (Excluding 0%), Li in a Content of 0.02% or Less (Excluding 0%), Pb in a Content of 0.5% or Less (Excluding 0%), and Bi in a Content of 0.5% or Less (Excluding 0%)
Calcium (Ca) contributes to spheroiclization of MnS and other sulfide inclusions and allows the steel to have better deformability and better machinability. To have such activities effectively, Ca may be contained in a content of preferably 0.0005% or more, and more preferably 0.001% or more. However, Ca, if contained in excess, may have saturated effects and is not expected to exhibit effects consistent with the content. To prevent this, the Ca content may be controlled to preferably 0.05% or less, more preferably 0.03% or less, and particularly preferably 0.01% or less.
As with Ca, the rare-earth element(s) (REM) contributes to spheroidization of MnS and other sulfide inclusions and allows the steel to have better deformability and better machinability. To have such activities effectively, REM may be contained in a content of preferably 0.0005% or more, and more preferably 0.001% or more. However, REM, if contained in excess, may have saturated effects and is not expected to exhibit effects consistent with the content. To prevent this, the REM content may be controlled to preferably 0.05% or less, more preferably 0.03% or less, and particularly preferably 0.01 percent by mass or less.
As used herein the term “REM” refers to element or elements including lanthanoid elements (fifteen elements from La to Lu), as well as Sc (scandium) and Y (yttrium). Of these elements, the steel sheet preferably contains at least one element selected from the group consisting of La, Ce, and Y and more preferably contains La and/or Ce as REM.
Magnesium (Mg) contributes to spheroidization of MnS and other sulfide inclusions and allows the steel to have better deformability and better machinability, as with Ca. To have such activities effectively, Mg may be contained in a content of preferably 0.0002% or more, and more preferably 0.0005% or more. However, Mg, if contained in excess, may have saturated effects and is not expected to exhibit effects consistent with the content. To prevent this, the Mg content may be controlled to preferably 0.02% or less, more preferably 0.015% or less, and particularly preferably 0.01% or less.
Lithium (Li) contributes to spheroidimtion of MnS and other sulfide indusions and allows the steel to have better deformability, as with Ca. In addition, this element allows aluminum oxides to have lower melting points and to be harmless and contributes to better machinability. To have such activities effectively, Li may be contained in a content of preferably 0.0002% or more, and more preferably 0.0005% or more. However, Li, if contained in excess, may have saturated effects and is not expected to exhibit effects consistent with the content. To prevent this, the Li content may be controlled to preferably 0.02% or less, more preferably 0.015% or less, and particularly preferably 0.01% or less.
Lead (Pb) effectively contributes to better machinability. To have such activities effectively, Pb may be contained in a content of preferably 0.005% or more, and more preferably 0.01% or more. However, Pb, if contained in excess, may cause problems upon production, such as formation of roll marks. To prevent this, the Pb content may be controlled to preferably 0.5% or less, more preferably 0.4% or less, and particularly preferably 0.3 percent by mass or less.
Bismuth (Bi) effectively contributes to better machinability, as with Pb. To have such activities effectively, Bi may be contained in a content of preferably 0.005% or more, and more preferably 0.01% or more. However, Bi, if contained in excess, may have saturated effects for better machinability. To prevent this, the Bi content may be controlled to preferably 0.5 percent by mass or less, more preferably 0.4% or less, and particularly preferably 0.3% or less.
Next, the microstructure featuring the steel sheet according to the present invention will be described.
Microstructure of Steel Sheet According to Present Invention
As is described above, the steel sheet according to the present invention is based on a steel including a ferrite-pearlite dual-phase microstructure, in which the size of ferrite grains is controlled within a specific range.
Pearlite in Content of Less than 20%, with Remainder Being Ferrite
The steel sheet according to the present invention includes a ferrite-pearlite dual-phase microstructure as its microstructure. Pearlite, if present in excess, may cause the steel sheet to have inferior formability. To prevent this, the content of pearlite may be controlled to less than 20%, more preferably 19% or less, furthermore preferably 18% or less, and particularly preferably 15% or less in area percentage. The remainder is approximately ferrite.
Ferrite Having Average Grain Size of from 3 to 35 μm
Ferrite grains constituting the ferrite phase may have an average grain size of 3 to 35 μm so as to allow the steel sheet to have better formability and satisfactory surface quality after forming. The steel sheet, if containing excessively fine (small) ferrite grains, may have excessively high deformation resistance. To prevent this, the average ferrite grain size may be controlled to 3 μm or more, preferably 4 μm or more, and more preferably 5 μm or more. In contrast, the steel sheet, if containing excessively coarse ferrite grains, may have inferior surface quality after forming and may have inferior properties such as toughness and fatigue properties. To prevent this, the average ferrite grain size may be controlled to 35 μm or less, preferably 30 μm or less, and more preferably 25 μm or less.
Method for Measuring Area Percentages of Phases
The area percentages of the individual phases may be determined by subjecting each test sample steel sheet to Nital etching, taking photos in five fields of view using a scanning electron microscope (SEM) at 1000-fold magnification, and determining proportions of ferrite and pearlite by point counting.
Method for Measuring Average Grain Size
The average ferrite grain size may be measured typically in the following manner. Specifically, sizes of ferrite grains present at three points, i.e., points corresponding to an outermost layer, one-fourth the thickness, and the central part in the thickness direction are measured. The grain size of one ferrite grain is measured in the following manner. The side surface in the rolling direction at each measurement point is subjected to Nital etching, photos in five fields of view in the portion are taken using a scanning electron microscope (SEM) at 1000-fold magnification, and the diameter including the center of gravity of a ferrite grain is determined by image analysis. The determined grain sizes are averaged to give an average ferrite grain size.
Next, a preferred method for producing the steel sheet according to the present invention will be illustrated below.
Preferred Method for Producing Steel Sheet According to Present Invention
The steel sheet according to the present invention may be produced by any method, as long as a material steel having the chemical composition can be formed into a desired thickness. Typically, the steel sheet may be produced by preparing a molten steel having the chemical composition in a converter, subjecting this to ingot making or continuous casting to give a slab, and rolling the slab into a hot-rolled steel sheet having a desired thickness, under following conditions.
Molten Steel Preparation
The nitrogen content in the molten steel can be adjusted by adding a raw material containing a nitrogen compound to the molten steel and/or controlling the atmosphere of the converter to be a nitrogen (N₂) atmosphere upon melting in the converter.
Heating
Heating before hot rolling is performed at a temperature of 1100° C. to 1300° C. The heating is performed at such a high temperature in order to dissolve nitrogen in an amount as much as possible while preventing the formation of nitrogen compounds. The lower limit of the heating temperature is preferably 1100° C., and more preferably 1150° C. In contrast, heating to a temperature higher than 1300° C. is operationally difficult.
Hot Rolling
Hot rolling is performed so that the finish rolling temperature be 880° C. or higher. The hot rolling, if performed at an excessively low finish rolling temperature, may cause ferrite transformation to occur at a high temperature, may thereby cause carbide precipitates in ferrite to coarsen, and may cause the steel sheet to have lower fatigue strength (inferior fatigue resistance). To prevent this, the hot rolling may be performed at a finish rolling temperature at a certain level or higher. The hot rolling may be performed at a finish rolling temperature of more preferably 900° C. or higher so as to allow austenite grains to coarsen and allow ferrite grains to have larger grain sizes to certain extent. The upper limit of the finish rolling temperature may be 1000° C., because such a high finish rolling temperature as to exceed 1000° C. is difficult to attain.
Hot Rolling Pass Schedule
The hot-rolled steel sheet according to the present invention has a thickness of 3 to 20 mm. The refinement of ferrite grains to control the average ferrite grain size within the predetermined range requires not only the control of the rolling temperature, but also the control of tandem rolling in the finish rolling to provide a final rolling reduction of 15% or more. In general, the finish rolling is performed as five to seven passes of tandem rolling. In this process, a pass schedule is determined from the viewpoint of control of holding fast with the rollers and the steel sheet, and the final rolling reduction is generally set at about 12% to 13% or more, preferably 16% or more, and more preferably 17% or more. With an increasing final rolling reduction (e.g., 20% to 30%), the hot rolling more effectively contributes to the grain refinement. However, the upper limit of the final rolling reduction may be set at about 30% from the viewpoint of rolling control.
Rapid Cooling After Hot Rolling
Within 5 seconds after the completion of the finish rolling, the workpiece is subjected to rapid cooling at a cooling rate (first rapid cooling rate) of 20° C./s or more, where the rapid cooling is stopped at a temperature (rapid cooling stop temperature) of from 580° C. to lower than 670° C. The rapid cooling is performed so as to obtain a ferrite-pearlite dual-phase microstructure having predetermined phase fractions. The rapid cooling, if performed at a rate (rapid cooling rate) of less than 20° C./s, may accelerate pearlite transformation. The rapid cooling, if stopped at a temperature of lower than 580° C., may accelerate pearlite transformation or bainite transformation. The rapid cooling in both cases may hardly give a ferrite-pearlite steel having predetermined phase fractions and may cause the steel sheet to have inferior bendability. In contrast, the rapid cooling, if stopped at a temperature of 670° C. or higher, may cause carbide precipitates in ferrite to coarsen and may cause the steel sheet to have deteriorated fatigue strength. The rapid cooling may be stopped at a temperature of preferably 600° C. to 650° C., and more preferably 610° C. to 640° C.
Slow Cooling After Rapid Cooling Stop
After the rapid cooling stop, the workpiece is slowly cooled by natural cooling or air cooling at a cooling rate (slow cooling rate) of 10° C./s or less for 5 to 20 seconds. This allows ferrite formation to proceed sufficiently and still contributes to appropriate refinement of carbide precipitates in ferrite. The slow cooling, if performed at a cooling rate greater than 10° C./s and/or for a cooling time shorter than 5 seconds, may cause insufficient formation of ferrite. In contrast, the slow cooling, if performed for a time longer than 20 seconds, may fail to allow carbide precipitates to coarsen and may cause the steel sheet to have deteriorated fatigue strength.
Rapid Cooling and Coiling After Slow Cooling
After the slow cooling, the workpiece is subjected again to rapid cooling at a cooling rate (second rapid cooling rate) of 20° C./s or more and coiled at a temperature of from higher than 300° C. to 450° C. This process is performed so as to allow the microstructure to mainly include ferrite and to allow the steel sheet to have sufficient bendability at certain level. The second rapid cooling, if performed at a cooling rate (second rapid cooling rate) of less than 20° C./s, or the coiling, if performed at a temperature of higher than 450° C., may cause the steel sheet to include an excessively large amount of pearlite. In contrast, the coiling, if performed at a temperature lower than 300° C., may cause the steel sheet to include martensite and retained austenite and to have inferior bendability.
The present invention will be illustrated in further detail with reference to several examples (experimental examples) below. It should be noted, however, that the examples are by no means intended to limit the scope of the invention; that various changes and modifications can naturally be made therein without deviating from the spirit and scope of the invention as described herein and that all such changes and modifications should be considered to be within the scope of the invention.

EXAMPLES

Steels having chemical compositions given in Table 1 below were made by vacuum melting, cast into ingots having a thickness of 120 mm, subjected to hot rolling under conditions given in Table 2, and yielded hot-rolled steel sheets. In each test (sample), rapid cooling after the completion of finish rolling down to the rapid cooling stop temperature was performed at a cooling rate of 20° C./s or more, and slow cooling after the rapid cooling stop was performed at a cooling rate of 10° C./s or less for 5 to 20 seconds.
The hot-rolled steel sheets obtained in the above manner were examined to determine the solute nitrogen content, area percentages of individual phases, and average ferrite grain size in the microstructures of the steel sheets by the measurement methods described as above in Description of Embodiments.
As the formability of the hot-rolled steel sheets, 90-degree bendability was evaluated by a 90-degree V-block test, because the steel sheets are those having a thickness of about 10 mm. In the test, a test specimen was pushed into a 90-degree die using a 90-degree punch, retrieved from the die, and the outside of the bent portion was visually observed. The punch has such a curvature that the ratio R/t of the punch inside minimum bend radius R to the steel sheet thickness t be 1. As a result of the visual observation, a sample suffering from fracture was evaluated as “×”; a sample not suffering from fracture, but suffering from an apparent crack was evaluated as “Δ”; a sample suffering from no crack although having fine asperities (wrinkles) was evaluated as “◯”; and a sample suffering from neither crack nor wrinkles was evaluated as “⊚”. As used herein the terms “fracture” and “crack” (cracking) refer respectively to one having a maximum gap distance of 1 mm or more and one having a maximum gap distance of less than 1 mm and are distinguished from each other.
The hardness of the surface in the bent portion after the bend test was measured to evaluate surface hardness after forming. The hardness was measured as a Vickers hardness (Hv) of each test specimen after forming using a Vickers hardness tester. The measurement was performed five times with a load of 1000 g, at a measurement position of the central part corresponding to one-fourth the diameter (D) of the resulting part.
The results of the measurements are indicated in Table 3 below.

TABLE 1

	Chemical composition (in mass percent) with the remainder consisting of Fe and
	inevitable impurities

Steel	C	Si	Mn	P	S	Al	N	10C + N	Others

a	0.02	0.02	0.40	0.007	0.001	0.025	0.011	0.21	—
b	0.05	0.02	0.40	0.007	0.001	0.022	0.008	0.51	—
c	0.05	0.02	0.40	0.007	0.001	0.022	0.023	0.52	—
d	0.05	0.10	0.30	0.007	0.001	0.023	0.009	0.51	—
e	0.05	0.40	0.20	0.007	0.001	0.024	0.009	0.51	—
f	0.10	0.02	0.40	0.007	0.001	0.022	0.010	1.01	—
g	0.15	0.02	0.40	0.007	0.001	0.024	0.009	1.51	—
h	0.20	0.02	0.40	0.007	0.001	0.022	0.010	2.01	—
i	0.26	0.02	0.40	0.007	0.001	0.023	0.009	2.61	—
j	0.05	0.02	0.40	0.007	0.001	0.025	0.003	0.50	—
k	0.05	0.02	0.40	0.007	0.001	0.025	0.030	0.53	—
l	0.31	0.02	0.40	0.007	0.001	0.025	0.008	3.11	—
m	0.05	0.60	0.40	0.007	0.001	0.025	0.010	0.51	—
n	0.05	0.02	0.15	0.007	0.001	0.025	0.012	0.51	—
o	0.05	0.02	1.10	0.007	0.001	0.025	0.011	0.51	—
p	0.05	0.02	0.40	0.060	0.001	0.025	0.010	0.51	—
q	0.05	0.02	0.40	0.007	0.060	0.025	0.011	0.51	—
r	0.05	0.02	0.40	0.007	0.001	0.005	0.012	0.51	—
s	0.05	0.02	0.40	0.007	0.001	0.11	0.013	0.51	—
t	0.05	0.02	0.40	0.007	0.001	0.025	0.010	0.51	Cr: 0.5, Mb: 0.03
u	0.05	0.02	0.40	0.007	0.001	0.025	0.010	0.51	Cu: 0.06, Ni: 0.15
v	0.05	0.02	0.40	0.007	0.001	0.025	0.009	0.51	Ca: 0.0025, Li: 0.001
w	0.05	0.02	0.40	0.007	0.001	0.025	0.009	0.51	Cr: 0.5, Mb: 0.03
x	0.30	0.02	0.40	0.007	0.001	0.024	0.025	3.03	—

(Element indicated with “—”: not added, underlined data: out of the scope of the present invention)

TABLE 2

	Hot rolling conditions

			Final		Rapid cooling		Thickness
		Heating	reduction	Final rolling	stop	Coiling	of hot-rolled
Production		temperature	rate	temperature	temperature	temperature	sheet
number	Steel	(° C.)	(%)	(° C.)	(° C.)	(° C.)	(mm)

1	a	1250	16	920	620	430	10
2	a	1250	18	911	607	405	4
3	a	1250	16	910	590	405	18
4*	a	1000*	15	780*	545*	320	10
5*	a	1250	15	900	600	410	25
6*	a	1250	9*	893	593	414	10
7	b	1250	18	920	649	399	10
8	c	1250	16	922	607	311	10
9	d	1250	16	903	593	311	10
10	e	1250	15	914	659	381	10
11	f	1250	16	923	594	395	10
12	g	1250	16	886	596	369	10
13	h	1250	17	891	634	352	10
14	i	1250	18	901	615	332	10
15	j	1250	16	894	617	346	10
16	k	1250	16	892	607	392	10
17	l	1250	17	911	594	331	10
18	m	1250	16	898	615	416	10
19	n	1250	17	913	612	320	10
20	o	1250	15	922	623	427	10
21	p	1250	16	928	636	325	10
22	q	1250	16	928	640	428	10
23	r	1250	17	910	616	374	10
24	s	1250	15	892	625	357	10
25	t	1250	17	896	613	368	10
26	u	1250	15	896	627	315	10
27	v	1250	15	929	624	391	10
28	w	1250	16	910	641	403	10
29	x	1250	17	907	605	411	10

(Underlined data: out of the scope of the present invention, asterisked data: out of the recommended range)

TABLE 3

	Microstructure	Surface

			Solute	Ferrite	Pearlite			hardness
			nitrogen	area	area	Average ferrite		after
Steel		Production	content	percentage	percentage	grain size	90-Degree	forming
No.	Steel	number	(mass %)	(%)	(%)	(μm)	bendability	(Hv)	Remarks

1	a	1	0.0085	98	2	29	⊚	260	Inventive steel sheet
2	a	2	0.008	94	6	16	⊚	270	Inventive steel sheet
3	a	3	0.008	98	2	33	◯	255	Inventive steel sheet
4	a	4*	0.003	96	4	30	⊚	180	Comparative steel sheet
5	a	5*	0.009	88	12	45	X	190	Comparative steel sheet
6	a	6*	0.008	92	8	41	Δ	190	Comparative steel sheet
7	b	7	0.007	97	3	24	◯	280	Inventive steel sheet
8	c	8	0.019	97	3	23	◯	305	Inventive steel sheet
9	d	9	0.0085	97	3	21	◯	283	Inventive steel sheet
10	e	10	0.008	96	4	21	◯	290	Inventive steel sheet
11	f	11	0.009	95	5	19	◯	307	Inventive steel sheet
12	g	12	0.008	90	10	13	◯	318	Inventive steel sheet
13	h	13	0.009	87	13	11	◯	332	Inventive steel sheet
14	i	14	0.008	83	17	9	◯	351	Inventive steel sheet
15	j	15	0.002	96	4	25	⊚	171	Comparative steel sheet
16	k	16	0.025	97	3	23	X	—	Comparative steel sheet
17	l	17	0.007	75	25	8	X	—	Comparative steel sheet
18	m	18	0.0085	96	4	24	X	—	Comparative steel sheet
19	n	19	0.010	97	3	23	◯	221	Comparative steel sheet
20	o	20	0.009	95	5	21	X	—	Comparative steel sheet
21	p	21	0.009	96	4	26	X	—	Comparative steel sheet
22	q	22	0.010	97	3	27	X	—	Comparative steel sheet
23	r	23	0.011	97	3	26	X	—	Comparative steel sheet
24	s	24	0.012	97	3	24	X	—	Comparative steel sheet
25	t	25	0.008	98	2	15	◯	290	Inventive steel sheet
26	u	26	0.009	97	3	16	◯	280	Inventive steel sheet
27	v	27	0.008	97	3	17	◯	270	Inventive steel sheet
28	w	28	0.008	98	2	15	◯	275	Inventive steel sheet
29	x	29	0.020	81	19	11	X	—	Comparative steel sheet

(Underlined data: out of the scope of the present invention, asterisked data: out of the recommended range, Evaluation in 90-degree bendability: ⊚: very good, ◯: good, Δ: surface crack, X: fracture, —: Not measured due to fracture, Inventive steel sheet: one having very good or good 90-degree bendability and a surface hardness after forming of 250 Hv or more, Comparative steel sheet: one not meeting the conditions for the inventive steel sheet)

As is indicated in Table 3, Steel Sheet Nos. 1 to 3, 7 to 14, and 25 to 28 employed steels having chemical compositions meeting the conditions specified in the present invention and were produced under recommended hot rolling conditions. As a result, these gave inventive steel sheets having microstructures meeting the conditions specified in the present invention. The steel sheets had 90-degree bendability and surface hardness after forming both meeting the acceptance criteria, demonstrating that the hot-rolled steel sheets have good cold formability during forming and still have predetermined surface hardness (strength) after forming.
In contrast, Steel Sheet Nos. 4 to 6, 15 to 24, and 29 are comparative steel sheets not meeting at least one of the conditions for the chemical composition and microstructure specified in the present invention. These steel sheets did not meet at least one of the 90-degree bendability and surface hardness after forming not meeting the acceptance criterion.
Typically, Steel Sheet No. 4 had a chemical composition meeting the condition, but underwent heating before hot rolling at an excessively low temperature out of the recommended range, and included solute nitrogen in an insufficient content. The steel sheet had poor surface hardness after forming.
Steel Sheet No. 5 had a chemical composition meeting the condition, but had an excessively large thickness after hot rolling out of the specific range. The steel sheet included coarsened ferrite grains and was inferior both in 90-degree bendability and in surface hardness after forming.
Steel Sheet No. 6 had a chemical composition meeting the condition, but underwent hot rolling with an excessively low final rolling reduction out of the recommended range. The steel sheet included coarsened ferrite grains and was inferior both in 90-degree bendability and in surface hardness after forming.
Steel Sheet No. 15 (Steel j) underwent hot rolling under conditions within the recommended range, but had an excessively low nitrogen content, and thereby had poor surface hardness after forming.
In contrast, Steel Sheet No. 16 (Steel k) underwent hot rolling under conditions within the recommended range, but had an excessively high nitrogen content, and was inferior at least in 90-degree bendability.
Steel Sheet No. 17 (Steel l) underwent hot rolling under conditions within the recommended range, but had an excessively high carbon content and failed to meet the condition as specified by the expression: 10C+N≦3.0. The steel sheet included an excessively large amount of pearlite and was inferior at least in 90-degree bendability.
Steel Sheet No. 18 (Steel m) underwent hot rolling under conditions within the recommended range, but had an excessively high Si content, and was inferior at least in 90-degree bendability.
Steel Sheet No. 19 (Steel n) underwent hot rolling under conditions within the recommended range, but had an excessively low Mn content, and was inferior at least in surface hardness after forming.
In contrast, Steel Sheet No. 20 (Steel o) underwent hot rolling under conditions within the recommended range, but had an excessively high Mn content, and was inferior at least in 90-degree bendability.
Steel Sheet No. 21 (Steel p) underwent hot rolling under conditions within the recommended range, but had an excessively high phosphorus content, and was inferior at least in 90-degree bendability.
Steel Sheet No. 22 (Steel q) underwent hot rolling under conditions within the recommended range, but had an excessively high sulfur content, and was inferior at least in 90-degree bendability.
Steel Sheet No. 23 (Steel r) underwent hot rolling under conditions within the recommended range, but had an excessively low Al content, and was inferior at least in 90-degree bendability.
In contrast, Steel Sheet No. 24 (Steel s) underwent hot rolling under conditions within the recommended range, but had an excessively high Al content, and was inferior at least in 90-degree bendability.
In contrast, Steel Sheet No. 29 (Steel x) underwent hot rolling under conditions within the recommended range, but failed to meet the condition as specified by the expression: 10C+N≦3.0, and was inferior at least in 90-degree bendability.
These results demonstrate the applicability of the present invention.
While the present invention has been particularly described with reference to specific embodiments thereof it is obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present invention.
The present application claims priority to Japanese Patent Application No. 2013-002640 filed on Jan. 10, 2013 and Japanese Patent Application No. 2013-056658 filed on Mar. 19, 2013, the entire contents of which are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

The hot-rolled steel sheets according to the present invention are suitable for automobile parts such as gears and other gearbox unit parts, and casings.

Claims

1. A hot-rolled steel sheet having excellent cold formability and satisfactory surface hardness after forming,

the hot-rolled steel sheet having a thickness of 3 to 20 mm,

the hot-rolled steel sheet comprising, in chemical composition

in percent by mass (hereinafter the same for chemical composition):

C in a content of 0.3% or less (excluding 0%);

Si in a content of 0.5% or less (excluding 0%);

Mn in a content of 0.2% to 1%;

Pin a content of 0.05% or less (excluding 0%);

S in a content of 0.05% or less (excluding 0%);

Al in a content of 0.01% to 0.1%;

N in a content of 0.008% to 0.025%;

with the remainder consisting of iron and inevitable impurities,

a content of solute nitrogen being 0.007% or more, and

the contents of carbon (C) and nitrogen (N) meeting a condition as specified by expression: 10C+N≦3.0,

a microstructure of the hot-rolled steel sheet comprising:

pearlite in a content of less than 20% in area percentage based on the total microstructure,

with the remainder approximately consisting of ferrite,

the ferrite having an average grain size of 3 to 35 μm.

2. The hot-rolled steel sheet according to claim 1,

further comprising, in the chemical composition, at least one element selected from the group consisting of:

Cr in a content of 2% or less (excluding 0%);

Mo in a content of 2% or less (excluding 0%);

Ti in a content of 0.2% or less (excluding 0%);

Nb in a content of 0.2% or less (excluding 0%)

V in a content of 0.2% or less (excluding 0%)

B in a content of 0.005% or less (excluding 0%);

Cu in a content of 5% or less (excluding 0%);

Ni in a content of 5% or less (excluding 0%);

Co in a content of 5% or less (excluding 0%);

Ca in a content of 0.05% or less (excluding 0%);

at least one rare-earth element (REM) in a total content of 0.05% or less (excluding 0%);

Mg in a content of 0.02% or less (excluding 0%);

Li in a content of 0.02% or less (excluding 0%);

Pb in a content of 0.5% or less (excluding 0%); and

Bi in a content of 0.5% or less (excluding 0%).