Patent Application
20200165708
This disclosure relates to a high-strength galvanized steel sheet suitable for automotive parts and having good bendability, spot weld crack resistance, stretch-flangeability, and axial compression stability (which means that a sheet can be stably pleated without cracks or fractures in axial compression) and to a method of producing the high-strength galvanized steel sheet.
From the perspective of improved crash safety, fuel efficiency, and rust prevention of automobiles, there is a demand for anticorrosive high-strength steel sheets for use in automotive parts. However, higher-strength materials often have poorer workability. Thus, there is a demand for steel sheets with high workability as well as high strength and rust prevention. Furthermore, processed parts should have high performance. For example, parts such as crash boxes having a bent portion or a punched surface should have good bendability and stretch-flangeability from the perspective of workability. From the perspective of assembly and performance of parts, parts should have good spot welds and stable axial compression characteristics. Those characteristics are desirably totally satisfied. Japanese Unexamined Patent Application Publication No. 2015-193907 discloses a technique related to a galvannealed steel sheet with good bendability and stretch-flangeability. However, Japanese Unexamined Patent Application Publication No. 2015-193907 does not describe spot weld cracking due to galvanizing or axial compression stability, and there is room for improvement in those characteristics.
Japanese Unexamined Patent Application Publication No. 2009-138222 discloses a technique for decreasing the size of ferrite grains to improve axial compression stability. However, Japanese Unexamined Patent Application Publication No. 2009-138222 does not consider stretch-flangeability, leaving room for improvement of stretch-flangeability. Furthermore, Japanese Unexamined Patent Application Publication No. 2009-138222 has no findings in galvanized steel sheets and does not consider spot weld cracking due to galvanizing.
Thus, no steel sheets totally satisfy high strength, bendability, spot weld crack resistance, stretch-flangeability, and axial compression stability.
It could therefore be helpful to provide a high-strength galvanized steel sheet with good bendability, spot weld crack resistance, stretch-flangeability, and axial compression stability and a method of producing the high-strength galvanized steel sheet.
We thus provide a high-strength hot-dip galvanized steel sheet that includes a steel sheet and a galvanized layer disposed on the steel sheet, wherein the steel sheet has a composition and a steel microstructure, the composition including, on a mass percent basis, C: 0.08% to 0.15%, Si: 0.1% to 1.0%, Mn: 2.0% to 3.0%, P: 0.100% or less (excluding 0%), S: 0.02% or less (excluding 0%), Al: 0.01% to 1.0%, and N: 0.010% or less, and one or more selected from Cr: 0.005% to 2.0%, Ti: 0.005% to 0.20%, Nb: 0.005% to 0.20%, Mo: 0.005% to 2.0%, and V: 0.005% to 2.0%, Cr+Ti+Nb+Mo+V being greater than or equal to Si, the remainder being Fe and incidental impurities, the steel microstructure containing ferrite and carbide-free bainite constituting 25% or less (including 0%) by area fraction in total, tempered martensite and carbide-containing bainite constituting 70% to 97% by area fraction in total, martensite and retained austenite constituting 3% to 20% by area fraction in total, and retained austenite constituting 1% to 5% by area fraction, the ferrite, the carbide-free bainite, the martensite, and the retained austenite constituting 3% to 30% by area fraction in total, the retained austenite having a C content of 0.10% to 0.50%, the carbide-containing bainite and the tempered martensite having an average grain size of 5 to 20 μm.
We further provide:
[1] A high-strength galvanized steel sheet including a steel sheet and a galvanized layer, wherein the steel sheet has a composition and a steel microstructure, the composition including, on a mass percent basis, C: 0.08% to 0.15%, Si: 0.1% to 1.0%, Mn: 2.0% to 3.0%, P: 0.100% or less (excluding 0%), S: 0.02% or less (excluding 0%), Al: 0.01% to 1.0%, and N: 0.010% or less, and one or more selected from Cr: 0.005% to 2.0%, Ti: 0.005% to 0.20%, Nb: 0.005% to 0.20%, Mo: 0.005% to 2.0%, and V: 0.005% to 2.0%, Cr+Ti+Nb+Mo+V being greater than or equal to Si, the remainder being Fe and incidental impurities, the steel microstructure containing ferrite and carbide-free bainite constituting 25% or less (including 0%) by area fraction in total, tempered martensite and carbide-containing bainite constituting 70% to 97% by area fraction in total, martensite and retained austenite constituting 3% to 20% by area fraction in total, and retained austenite constituting 1% to 5% by area fraction, the ferrite, the carbide-free bainite, the martensite, and the retained austenite constituting 3% to 30% by area fraction in total, the retained austenite having a C content of 0.10% to 0.50%, the carbide-containing bainite and the tempered martensite having an average grain size of 5 to 20 μm, and the galvanized layer is disposed on the steel sheet and has an Fe content of 6% or more by mass.
[2] The high-strength galvanized steel sheet according to [1], wherein the composition further includes, on a mass percent basis, one or more selected from Ni: 0.005% to 2.0%, Cu: 0.005% to 2.0%, B: 0.0001% to 0.0050%, Ca: 0.0001% to 0.0050%, REM: 0.0001% to 0.0050%, Sn: 0.01% to 0.50%, and Sb: 0.0010% to 0.10%.
[3] A method of producing a high-strength galvanized steel sheet, including: an annealing step of hot-rolling and pickling a steel having a composition according to [1] or [2] to form a hot-rolled sheet, optionally cold-rolling the hot-rolled sheet at a rolling reduction of 20% or more to form a cold-rolled sheet, heating the hot-rolled sheet or the cold-rolled sheet to a heating temperature in the range of 780° C. to 950° C., a heating rate in a temperature range of 600° C. to 720° C. being 3.0° C./s or less, cooling the sheet to a temperature in the range of 150° C. to 400° C., an average cooling rate in a temperature range of 450° C. to 720° C. being 10° C./s or more, a residence time in the temperature range of 780° C. to 950° C. in the heating and cooling being 30 seconds or more and, after the cooling, reheating the sheet at a heating temperature of more than 150° C. and 600° C. or less, a residence time at a temperature of more than 150° C. and 600° C. or less being 1000 seconds or less, or after the cooling, holding the sheet at a temperature of more than 150° C. and 400° C. or less for a residence time of 1000 seconds or less; and a galvanizing step of galvanizing the annealed sheet after the annealing step, then heating the sheet to a temperature in the range of 460° C. to 600° C. for plating and alloying treatment, and cooling the sheet to 50° C. or less.
We provide a high-strength galvanized steel sheet with good bendability, spot weld crack resistance, stretch-flangeability, and axial compression stability. The high-strength galvanized steel sheet is suitable as a material for automotive parts.
Examples of our steel sheets and methods will be described below. This disclosure is not limited to the examples.
Our high-strength galvanized steel sheet includes a steel sheet and a galvanized layer. First, the steel sheet will be described below. The steel sheet has a particular composition and a particular steel microstructure. The composition and the steel microstructure are described below in this order.
The steel sheet has a composition including, on a mass percent basis, C: 0.08% to 0.15%, Si: 0.1% to 1.0%, Mn: 2.0% to 3.0%, P: 0.100% or less (excluding 0%), S: 0.02% or less (excluding 0%), Al: 0.01% to 1.0%, and N: 0.010% or less, and one or more selected from Cr: 0.005% to 2.0%, Ti: 0.005% to 0.20%, Nb: 0.005% to 0.20%, Mo: 0.005% to 2.0%, and V: 0.005% to 2.0%, Cr+Ti+Nb+Mo+V being greater than or equal to Si, the remainder being Fe and incidental impurities.
The composition may further include, on a mass percent basis, one or more selected from Ni: 0.005% to 2.0%, Cu: 0.005% to 2.0%, B: 0.0001% to 0.0050%, Ca: 0.0001% to 0.0050%, REM: 0.0001% to 0.0050%, Sn: 0.01% to 0.50%, and Sb: 0.0010% to 0.10%. Each of the components will be described below. The symbol “%” in the component content refers to “% by mass”.
C is an effective element that contributes to an increased area fraction, strength, and axial compression stability of martensite or retained austenite. C content of less than 0.08% is insufficient for such effects. C content of more than 0.15% results in deteriorated bendability or spot weld cracking. Thus, C content is 0.08% to 0.15%. The lower limit is preferably 0.09% or more. The upper limit is preferably 0.13% or less.
Si reduces formation of carbide and promotes concentration of C in austenite, thereby inducing formation of retained austenite. Thus, Si is an effective element that contributes to improved axial compression stability. Such effects require a Si content of 0.1% or more. Si content of more than 1.0% results in excessive stabilization of retained austenite and low axial compression stability. Thus, Si content is 0.1% to 1.0%. The lower limit is preferably 0.15% or more, more preferably more than 0.3%. The upper limit is preferably 0.7% or less.
Mn is an element that is effective in inhibiting formation of ferrite or carbide-free bainite and forming tempered martensite or retained austenite. Mn content of 2.0% or less is insufficient for such effects. Mn content of more than 3.0% results in spot weld cracking. Thus, Mn content is 2.0% to 3.0%. The lower limit is preferably 2.3% or more. The upper limit is preferably 2.8% or less.
P lowers hole expandability, and the P content is desirably minimized. The P content may be 0.100% or less. Although P content has no lower limit, P content of less than 0.001% results in low production efficiency. Thus, P content is preferably 0.001% or more.
S lowers hole expandability, and S content is preferably minimized. S content may be 0.02% or less. Although S content has no lower limit, S content of less than 0.0002% results in low production efficiency. Thus, S content is preferably 0.0002% or more.
Al functions as a deoxidizer and is preferably added in a deoxidation step. Such an effect requires Al content of 0.01% or more. Al content of more than 1.0% results in a large amount of ferrite or carbide-free bainite, which is unfavorable. Thus, Al content is 0.01% to 1.0%. The lower limit is preferably 0.02% or more. The upper limit is preferably 0.50% or less.
An excessive amount of N results in spot weld cracking or deteriorated bendability. Thus, N content is 0.010% or less. The lower limit is preferably 0.0010% or more. The upper limit is preferably 0.0060% or less.
At Least One Selected from Cr: 0.005% to 2.0%, Ti: 0.005% to 0.20%, Nb: 0.005% to 0.20%, Mo: 0.005% to 2.0%, and V: 0.005% to 2.0%
Cr, Nb, Ti, Mo, and V form carbides and decrease the C content of retained austenite. Thus, these elements are important components that contribute to improved axial compression stability. Cr, Nb, Ti, Mo, or V content lower than the lower limit described above results in low axial compression stability. Cr, Nb, Ti, Mo, or V content higher than the upper limit results in an excessively low C content of retained austenite and a low hole expanding ratio. Thus, at least one selected from Cr, Nb, Ti, Mo, and V contents is Cr: 0.005% to 2.0%, Nb: 0.005% to 0.20%, Ti: 0.005% to 0.20%, Mo: 0.005% to 2.0%, or V: 0.005% to 2.0%. The preferred lowest contents are 0.10% or more for Cr, 0.01% or more for Nb, 0.01% or more for Ti, 0.05% or more for Mo, and 0.03% or more for V. The preferred highest contents are 0.80% or less for Cr, 0.05% or less for Nb, 0.05% or less for Ti, 0.50% or less for Mo, and 0.50% or less for V.
For two or more elements selected from Cr, Nb, Ti, Mo, and V, the amount of at least one of the elements is in the range described above. This is because the amounts of the other elements below the lower limits do not reduce the desired effects. Components in amounts below the lower limits are considered to be incidental impurities.
The amount of Cr, Ti, Nb, Mo, and V, which are carbide-forming elements, greater than or equal to the amount of Si, which is a carbide-inhibiting element, results in a low C content of retained austenite and desired axial compression stability. Thus, the sum of the Cr, Ti, Nb, Mo, and V contents is greater than or equal to the Si content.
The remainder includes Fe and incidental impurities. If necessary, the composition may include an appropriate amount of one or more of the following elements.
As described above, the composition may further include one or more selected from Ni: 0.005% to 2.0%, Cu: 0.005% to 2.0%, B: 0.0001% to 0.0050%, Ca: 0.0001% to 0.0050%, REM: 0.0001% to 0.0050%, Sn: 0.01% to 0.50%, and Sb: 0.0010% to 0.10%.
Ni, Cu, and B are effective elements that induce formation of martensite and that contribute to high strength. Ni, Cu, and B contents are above the lower limits to effectively produce such effects. Ni, Cu, or B content above the upper limit results in spot weld cracking or deteriorated axial compression stability. The preferred lowest contents are 0.050% or more for Ni, 0.050% or more for Cu, and 0.0005% or more for B. The preferred highest contents are 0.5% or less for Ni, 0.5% or less for Cu, and 0.0040% or less for B.
Ca and REM are effective elements that contribute to improved hole expandability due to the morphology control of inclusions. The amounts of these elements are above the lower limits to effectively produce such an effect. Ca content or a REM content of more than 0.0050% may result in poor hole expandability due to an increased amount of inclusions. The preferred lowest contents are 0.0005% or more for Ca and 0.0005% or more for REM. The preferred highest contents are 0.0030% or less for Ca and 0.0030% or less for REM.
Sn and Sb are elements that inhibit, for example, decarbonization, denitrification, or deboronization and are effective in inhibiting strength reduction of steel. Sn and Sb are also effective in inhibiting spot weld cracking. The amounts of these elements are above the lower limits to effectively produce such effects. Sn content or Sb content above the upper limit may result in poor hole expandability. The preferred lowest content is 0.0050% or more for each of Sn and Sb. The preferred highest contents are 0.10% or less for Sn and 0.05% or less for Sb.
Ni, Cu, B, Ca, REM, Sn, and Sb contents below their respective lower limits are permissible because these do not reduce the desired effects. Below the lower limits, these components are considered to be incidental impurities.
Impurity elements such as Zr, Mg, La, and Ce may constitute up to 0.002% as incidental impurities.
The steel microstructure is described below. The steel microstructure contains ferrite and carbide-free bainite constituting 25% or less (including 0%) by area fraction in total, tempered martensite and carbide-containing bainite constituting 70% to 97% by area fraction in total, martensite and retained austenite constituting 3% to 20% by area fraction in total, and retained austenite constituting 1% to 5% by area fraction. The ferrite, the carbide-free bainite, the martensite, and the retained austenite constitute 3% to 30% by area fraction in total. The retained austenite has a C content of 0.10% to 0.50%. The carbide-containing bainite and the tempered martensite have an average grain size of 5 to 20 μm. Bainite includes tempered bainite.
Ferrite and carbide-free bainite are unfavorable in terms of the balance between strength and stretch-flangeability and in terms of axial compression stability. Ferrite and carbide-free bainite can constitute up to 25% by area fraction in total. Thus, the total area fraction of ferrite and carbide-free bainite is 25% or less, preferably 15% or less. The area fraction is determined by the measurement method described in the examples.
Carbide-free bainite contains no carbide in an image data, which is taken with a scanning electron microscope (SEM) at a magnification of 1500 at one fourth the thickness of a sheet from a surface of the sheet in a cross section of the sheet parallel to the rolling direction and parallel to the thickness direction polished and then subjected to corrosion with 3% nital. In image data shown in
Tempered martensite and carbide-containing bainite are important for the balance between strength and stretch-flangeability. A total area fraction of these components below 70% results in an undesired tensile strength (TS) or hole expanding ratio. A total area fraction of more than 97% results in low axial compression stability due to a decreased amount of retained austenite or martensite. Thus, the total area fraction of tempered martensite and carbide-containing bainite is 70% to 97%. The lower limit is preferably 75% or more, more preferably 80% or more. The upper limit is preferably 95% or less, more preferably 93% or less. The area fraction is determined by the measurement method described in the examples.
Carbide-containing bainite contains carbide in an image data, which is taken with a scanning electron microscope (SEM) at a magnification of 1500 at one fourth the thickness of a sheet from a surface of the sheet in a cross section of the sheet parallel to the rolling direction and parallel to the thickness direction polished and then subjected to corrosion with 3% nital. The area fraction is determined by the method described in the examples.
A total area fraction of martensite (which means as-quenched martensite and also includes autotempered martensite) and retained austenite below 3% results in low axial compression stability. A total area fraction of more than 20% results in a low hole expanding ratio. Thus, the total area fraction of martensite and retained austenite is 3% to 20%. The lower limit is preferably 5% or more. The upper limit is preferably 15% or less. The area fraction is determined by the measurement method described in the examples. Retained austenite has a C content of 0.10% to 0.50%, as described below.
A minute amount of retained austenite is important for axial compression stability. Although the mechanism is not entirely clear, we believe that unstable retained austenite in a deformation stress concentrated portion changes immediately into martensite in axial compression and promotes strain propagation. Less than 1% of unstable retained austenite is insufficient for such an effect. More than 5% of unstable retained austenite lowers bendability or stretch-flangeability. Thus, the area fraction of retained austenite is 1% to 5%. The area fraction is determined by the measurement method described in the examples.
C content of retained austenite above 0.50% results in excessively stable retained austenite and low axial compression stability. C content of less than 0.10% results in a low hole expanding ratio. Thus, C content of retained austenite is 0.10% to 0.50%. The lower limit is preferably 0.2% or more. The upper limit is preferably 0.45% or less. The C content is determined by the measurement method described in the examples.
The total area fraction of ferrite, carbide-free bainite, martensite, and retained austenite, if present at all, is 3% to 30% in terms of the balance between bendability, axial compression stability, and stretch-flangeability. A total area fraction of less than 3% results in undesired amounts of martensite and retained austenite and low axial compression stability. A total area fraction of more than 30% results in low bendability and a low hole expanding ratio. Thus, the total area fraction of ferrite, carbide-free bainite, martensite, and retained austenite is 3% to 30%. The lower limit is preferably 4% or more, more preferably 5% or more. The upper limit is preferably 15% or less, more preferably 12% or less.
An average grain size of tempered martensite or carbide-containing bainite below 5 μm results in an increased amount of retained austenite at grain boundaries and results in low bendability or a low hole expanding ratio. An average grain size of tempered martensite or carbide-containing bainite above 20 μm results in low bendability or axial compression stability. Thus, each of tempered martensite and carbide-containing bainite has an average grain size of 5 to 20 μm. The lower limit is preferably 6 μm or more. The upper limit is preferably 16 μm or less. The average grain size is determined by the method described in the examples.
The steel microstructure may include other microstructures such as pearlite. Other microstructures may constitute up to 10% by area fraction without reducing the desired effects. In other words, the phases described above preferably constitute 90% or more.
Next, the galvanized layer is described below. The galvanized layer has an Fe content of 6% or more by mass. Fe content of 6% or more by mass can result in improved spot weld crack resistance. The Fe content has no upper limit but is preferably 15% or less by mass in consideration of powdering.
A method of producing a high-strength galvanized steel sheet includes an annealing step and a galvanizing step.
The annealing step includes hot-rolling and pickling a steel having the composition described above to form a hot-rolled sheet, optionally cold-rolling the hot-rolled sheet at a rolling reduction of 20% or more to form a cold-rolled sheet, heating the hot-rolled sheet or the cold-rolled sheet to a heating temperature of 780° C. to 950° C., the heating rate in the temperature range of 600° C. to 720° C. being 3.0° C./s or less, cooling the sheet to a temperature of 150° C. to 400° C., the average cooling rate in the temperature range of 450° C. to 720° C. being 10° C./s or more, the residence time in the temperature range of 780° C. to 950° C. in the heating and cooling being 30 seconds or more, and after the cooling, reheating the sheet at a heating temperature of more than 150° C. and 600° C. or less, the residence time at a temperature of more than 150° C. and 600° C. or less being 1000 seconds or less, or after the cooling, holding the sheet at a temperature of more than 150° C. and 400° C. or less for a residence time of 1000 seconds or less.
First, a hot-rolled sheet for use in a production method will be described below. The hot-rolled sheet may be produced by any method. A slab for use in the production of the hot-rolled sheet is preferably formed by a continuous casting process to prevent macrosegregation or may be formed by an ingot casting process or a thin slab casting process. Prior to hot-rolling the slab, the slab may be cooled to room temperature and subsequently reheated for hot rolling or may be placed in a furnace for hot rolling without being cooled to room temperature. Alternatively, an energy-saving process may also be performed in which the slab is immediately hot-rolled after lightly conserving heat. The slab is preferably heated to 1100° C. or more to melt carbide or prevent an increase in rolling force. The slab is preferably heated to 1300° C. or less to prevent an increase in scale loss. The slab heating temperature is the surface temperature of the slab. Prior to hot-rolling the slab, a rough bar after rough rolling may be heated. A continuous rolling process may also be performed in which rough bars are joined together and continuously subjected to finish rolling. Finish rolling may increase anisotropy and reduce workability after cold rolling and annealing. Thus, the finishing temperature is preferably higher than the Ara transformation temperature. To reduce rolling force or make the shape and material quality uniform, lubrication rolling to achieve a friction coefficient of 0.10 to 0.25 is preferably performed in all or part of the finish rolling passes.
A steel sheet after coiling is pickled to remove scales and, if necessary, is subjected to heat treatment and/or cold rolling at a rolling reduction of 20% or more.
A rolling reduction in cold rolling below 20% results in the absence of the steel microstructure due to coarse grains formed by annealing. Thus, the rolling reduction in cold rolling is 20% or more, preferably 30% or more. Although the rolling reduction has no upper limit, a rolling reduction of more than 90% results in an undesired shape of the rolled sheet. Thus, a rolling reduction of 90% or less is preferred.
A heating rate of more than 3.0° C./s in the temperature range of 600° C. to 720° C. results in the absence of a steel microstructure due to a decrease in grain size. Thus, the heating rate in the temperature range of 600° C. to 720° C. is 3.0° C./s or less. The heating rate has no lower limit but is preferably 0.1° C./s or more in terms of, for example, production efficiency. Outside this temperature range, any heating rate is permissible. The heating rate is typically 0.5° C./s to 3.0° C./s. The heating rate refers to the average heating rate.
A heating temperature (annealing temperature) of less than 780° C. results in an undesired steel microstructure due to excessive formation of ferrite. A heating temperature of more than 950° C. results in an undesired steel microstructure due to coarse austenite. Thus, the heating temperature is 780° C. to 950° C., preferably 780° C. to 900° C.
An average cooling rate of less than 10° C./s in the temperature range of 450° C. to 720° C. results in an undesired steel microstructure due to formation of a large amount of ferrite or carbide-free bainite. Thus, the average cooling rate in the temperature range of 450° C. to 720° C. is 10° C./s or more, preferably 15° C./s or more. Although the average cooling rate has no upper limit, an average cooling rate of more than 100° C./s tends to result in considerable temperature variations. Thus, the average cooling rate is preferably 100° C./s or less from the viewpoint of operation. Outside this temperature range, any average cooling rate is permissible. The average cooling rate is typically 15° C./s to 40° C./s.
A cooling stop temperature of less than 150° C. results in an undesired steel microstructure due to formation of a decreased amount of retained austenite. A cooling stop temperature of more than 400° C. results in an undesired steel microstructure due to formation of an increased amount of martensite or retained austenite. Thus, the cooling stop temperature is 150° C. to 400° C., preferably 150° C. to 350° C. Cooling stop and retention hardly affects the final characteristics and may be performed as required.
A residence time of less than 30 seconds in the temperature range of 780° C. to 950° C. in the heating and cooling results in an undesired steel microstructure due to insufficient formation of austenite and due to formation of an increased amount of ferrite. Thus, the residence time in the temperature range of 780° C. to 950° C. is 30 seconds or more. The residence time has no upper limit but is preferably 1000 seconds or less in terms of, for example, production efficiency.
Reheating Temperature: More than 150° C. and 600° C. or Less
A reheating temperature of more than 600° C. results in an undesired amount of retained austenite due to the decomposition of austenite into pearlite or the like. This also results in softening of tempered martensite or carbide-containing bainite, thus resulting in low strength. Because heating to a temperature lower than the cooling stop temperature is impossible, the lower limit is more than 150° C. Thus, the reheating temperature is more than 150° C. and 600° C. or less, preferably more than 150° C. and 500° C. or less.
Residence Time at a Temperature of More than 150° C. and 600° C. or Less: 1000 Seconds or Less
A residence time of more than 1000 seconds results in an undesired amount of retained austenite due to decomposition of austenite into pearlite or the like. Thus, the residence time is 1000 seconds or less, preferably 500 seconds or less. The residence time is preferably 5 seconds or more in terms of operational stability.
Instead of reheating, after cooling, the sheet may be held at a temperature of more than 150° C. and 400° C. or less for a residence time of 1000 seconds or less.
The galvanizing step after the annealing step includes galvanizing an annealed sheet, then heating the sheet to a temperature of 460° C. to 600° C. for plating and alloying treatment, and cooling the sheet to 50° C. or less. For example, a coated layer formed on the annealed sheet contains, on a mass percent basis, Fe: 5.0% to 20.0%, Al: 0.001% to 1.0%, and one or two or more selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM: 0% to 30% in total, the remainder being Zn and incidental impurities. The plating treatment method can be hot-dip galvanizing under appropriate conditions.
In the alloying treatment after the galvanizing treatment, an alloying treatment temperature of less than 460° C. results in spot weld cracking due to insufficient alloying. An alloying temperature of more than 600° C. results in an undesired amount of retained austenite due to the decomposition of austenite into pearlite or the like. Thus, the alloying treatment temperature is 460° C. to 600° C., preferably 470° C. to 580° C. The alloying time is typically, but not limited to, 1 to 120 seconds.
After the alloying treatment, the sheet is cooled to 50° C. or less. Cooling to 50° C. or less changes austenite partly into retained austenite and partly into martensite, thus forming the desired microstructure.
A steel with a composition listed in Table 1 was melted in a vacuum melting furnace in a laboratory and was rolled to form a steel slab (in Table 1, N is an incidental impurity). The steel slab was heated to 1200° C., rough-rolled, and finish-rolled to form a hot-rolled sheet 3.0 mm in thickness. The finish-rolling temperature in the hot rolling was 900° C., and the coiling temperature was 500° C. The hot-rolled sheet was then pickled and optionally cold-rolled to form a cold-rolled sheet 1.4 mm in thickness (the rolling reduction is listed in Table 2). The hot-rolled sheet or cold-rolled sheet was annealed. The annealing was performed in a heat treatment and plating treatment apparatus in a laboratory under the conditions shown in Table 2. Thus, galvannealed steel sheets 1 to 31 were formed. In galvanizing treatment, the sheet was immersed in a plating bath at 460° C. and plated at 35 to 45 g/m2. In alloying treatment, the plated sheet was held at a temperature of 450° C. to 650° C. for 30 seconds and cooled to 50° C. or less. The galvannealed steel sheet was subjected to temper rolling at an elongation percentage of 0.3% and subjected to microstructure observation by the following test method. The galvannealed steel sheet was examined for tensile properties, bendability, hole expandability, spot weld crack resistance, and axial compression stability.
The area fractions of ferrite, carbide-free bainite, martensite, tempered martensite, and carbide-containing bainite refer to the area percentages of respective microstructures based on the observed area. These area fractions were determined by cutting a sample from the annealed steel sheet, taking image data in three visual fields with a scanning electron microscope (SEM) at a magnification of 1500 at one fourth the thickness of the sample in a cross section of the sample parallel to the rolling direction and parallel to the thickness direction polished and then subjected to corrosion with 3% nital, determining the area fraction of each microstructure using Image-Pro available from Media Cybernetics, and taking the average area fraction of the visual fields as the area fraction of each microstructure. In image data, ferrite is black, martensite and retained austenite are white or light gray, and bainite, including aligned carbide and/or island martensite, is black or dark gray (Observation of grain boundaries between bainite grains can differentiate between carbide-free bainite and carbide-free bainite. Island martensite is white or light gray in image data, as shown in
The volume fraction of retained austenite was determined by grinding an annealed steel sheet by one fourth the thickness of the sheet, chemically polishing the surface by 0.1 mm, measuring the integrated reflection intensities of the (200), (220), and (311) planes of fcc iron (austenite) and the (200), (211), and (220) planes of bcc iron (ferrite) with an X-ray diffractometer using Mo Kα radiation, and calculating the volume fraction from the intensity ratio of the integrated reflection intensities of the planes of the fcc iron to the integrated reflection intensities of the planes of the bcc iron. The volume fraction was considered to be the area fraction of retained austenite.
The lattice constant of retained austenite is calculated using formula (1) from the diffraction peak shift of the (220) plane with an X-ray diffractometer using Co Kα radiation. The C content of retained austenite is calculated using formula (2).
α=0.17889√{square root over (2)}/sin θ (1)
a=0.3578+0.0033[C]+0.000095[Mn]+0.00006[Cr]+0.0022[N]+0.00056[Al]+0.00015[Cu]+0.00031[Mo] (2)
In the formulae, a denotes the lattice constant (nm) of retained austenite phase, θ denotes the diffraction peak angle of the (220) plane divided by 2 (rad), and [M] denotes the mass percent of the element M in the retained austenite phase. The mass percent of the element M other than C in the retained austenite phase is based on the total mass of steel.
The Fe content of the galvanized layer was determined by dissolving the galvanized layer in hydrochloric acid containing an inhibitor and measuring the Fe content of the hydrochloric acid by ICP analysis.
A JIS 5 tensile test piece (JIS Z 2201) was taken from an annealed sheet in a direction perpendicular to the rolling direction. TS was measured at a strain rate of 10−3/s in a tensile test according to JIS Z 2241. 980 MPa or more is acceptable.
A strip of test specimen 30 mm in width and 100 mm in length was taken from an annealed sheet. A bending test was performed in the direction parallel to the rolling direction. A 90-degree V bending test was performed at a stroke rate of 10 mm/s, an indentation load of 10 tons, a press holding time of 5 seconds, and a bend radius of 3.5 mm for a test specimen 1.4 mm in thickness or 7.5 mm for a test specimen 3.0 mm in thickness. A bend line was observed with a 10× magnifying lens. A test specimen without 0.5-mm or larger cracks was rated “good”.
A test specimen 120 mm in width and 78 mm in length and a test specimen 120 mm in width and 150 mm in length were taken from an annealed sheet. The width direction is perpendicular to the rolling direction. The bend radius was 4 mm for a test specimen 1.4 mm in thickness or 8 mm for a test specimen 3.0 mm in thickness. An axial compression component illustrated in
Subsequently, as illustrated in
Two test specimens 50 mm in width and 150 mm in length were taken from an annealed sheet in the direction parallel to the rolling direction. As illustrated in
Stretch-flangeability was evaluated by subjecting a 150 mm×150 mm test specimen three times to a hole-expanding test according to JFS T 1001 (the Japan Iron and Steel Federation standard, 2008) with a 60-degree conical punch to determine the average hole expanding ratio (%). A hole expanding ratio of 40% or more was good.
Table 3 shows the results.
−0.185
0.05
0.16
1.10
0.00
1.5
3.2
10
770
980
500
650
1200
7
450
620
K
L
M
N
O
P
Q
20.7
20.9
41
23
36
77
23.1
21.9
42
25
33
75
47
18
35
82
33
67
100
100
0
0
0
0
1
1
6
4.3
3.9
0
0
0.54
2
0.53
30
59
41
Poor
Poor
23
Poor
Poor
Poor
24
Poor
34
Poor
38
Poor
Poor
Poor
Poor
Poor
38
Poor
Poor
Poor
Poor
Poor
Poor
Poor
35
Poor
The galvanized steel sheets according to the examples had TS of 980 MPa or more and had good bendability, spot weld crack resistance, stretch-flangeability, and axial compression stability. The Comparative examples had undesired strength, poor bendability, poor spot weld crack resistance, poor stretch-flangeability, or poor axial compression stability.
We provide a galvanized steel sheet with TS of 980 MPa or more, good bendability, spot weld crack resistance, stretch-flangeability, and axial compression stability. The use of a high-strength galvanized steel sheet in automotive parts can contribute greatly to improved crash safety and fuel efficiency of automobiles.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2016-023403 | Feb 2016 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2017/003151 | 1/30/2017 | WO | 00 |
