SILICON ALLOYED STEEL FOR HOT ROLLING

Abstract

A silicon alloyed steel for hot rolling and a process of hot rolling the silicon alloyed steel is provided. The process includes providing a steel slab having a chemical composition in weight percent within a range of 0.06-0.30 C, 0.3-2.0 Mn, 0.6-3.5 Si, and Fe plus incidental melting impurities. The steel slab is hot rolled and hot rolled steel strip is produced. The hot rolled steel strip is coiled at temperatures between 100-600° C. and has a microstructure containing at least 90 vol % ferrite plus pearlite, a yield strength of at least 400 megapascals (MPa), a tensile strength of at least 600 MPa, and a tensile elongation of at least 20%.

Description

BACKGROUND OF THE INVENTION

Hot rolled high strength structural steels containing not more than 0.22 weight percent (wt %) carbon (C) in combination with not more than 1.70 wt % manganese (Mn) and less than 0.3 wt % silicon (Si), and exhibiting strength levels of 350 megapascals (MPa) yield strength and 520 MPa tensile strength are known. Higher strength levels can be obtained with the addition of microalloying elements such as niobium (Nb), vanadium (V), and/or titanium (Ti). However, it is appreciated that the use of such microalloying elements increases the cost of the alloy. In addition, such alloys are known to have a high sensitivity to hot rolling processing parameters. In particular, coiling temperatures must be within +/−30° C. in order to obtain acceptable variations in the mechanical properties of the hot strip produced from such alloys.

In addition to the above, multiphase or complex phase steels have been proposed for serving or meeting desired high strength levels that exceed maximum levels of conventional high strength steels. However, such steels require not only accurate coiling temperatures but demand a specific time-temperature regime during finishing rolling, travel along a run-out table prior to coiling and during cooling. For example, hot rolled transformation-induced plasticity (TRIP) steels are not commercially available due to their strong sensitivity to hot strip processing temperature variations.

In contrast, fully martensitic steels can be easily processed if cooling capacity is sufficient to provide coiling temperatures of less than 200° C. Such steels can have tensile strengths of 1000 MPa and greater; however, ductility of these martensitic steels is typically less than 10%.

Given the above, an alloy for hot rolling that provides high strength and high ductility and yet is relatively insensitive to hot strip processing variations would be desirable.

SUMMARY OF THE INVENTION

A process for making a hot rolled strip from silicon alloyed steel is provided. The process includes providing a steel slab having a chemical composition in weight percent within a range of 0.06-0.30 carbon (C), 0.3-2.0 manganese (Mn), 0.6-3.5 silicon (Si), and iron (Fe) plus incidental melting impurities. The steel slab is hot rolled and hot rolled steel strip is produced. The hot rolled steel strip is coiled at temperatures between 600-100° C., has a microstructure containing at least 90 volume percent (vol. %) ferrite plus pearlite, a yield strength of at least 400 megapascals (MPa), a tensile strength of at least 600 MPa, and a tensile elongation of at least 20%. In addition, the coiled hot rolled strip has the above stated microstructure and mechanical properties irrespective of the coiling temperature between 100-600° C.

In some instances, the chemical composition of the steel slab can have up to 0.1 phosphorus (P), 0.08 maximum (max) aluminum (Al), 0.6 max chromium (Cr), 0.3 max nickel (Ni), 0.6 max copper (Cu), 0.08 max niobium (Nb), 0.6 max molybdenum (Mo), 0.10 max titanium (Ti), and 0.015 max sulfur (S). The steel alloy may or may not have a Si/Mn ratio between 1.3-3.5. In addition, the steel alloy can have a Si/Mn ratio between 0.8-3.5 when 0.03-0.05 P is present. In the alternative, the steel alloy can have a Si/Mn ratio between 0.3-3.5 when 0.05-0.1 P is present.

In some instances, the steel slab is hot rolled in an austenitic finishing mode and cooled at a cooling rate between 10-100 Kelvin per second (K/s) when the hot rolled strip has exited the last hot rolling stand and is between 950° C. and 400° C. at the onset of such accelerated cooling. In addition, the hot rolled strip can be subjected to a cooling interruption within a range of 0-20 seconds, e.g. within 3-20 seconds, when the hot rolled strip is between a finishing rolling station and coiling. In the event that the hot rolled strip is subjected to the cooling interruption, the strip is cooled at the cooling rate between 10-100 K/s before or after the cooling interruption and when the strip is at temperatures between 950-400° C.

In other instances, the steel slab is hot rolled in a partially ferritic finishing rolling mode and cooled at a cooling rate between 10-100 K/s when the hot rolled strip is between 850-400° C. at the onset of such accelerated cooling. Similar to hot rolling of the steel strip in the austenitic finishing roll mode, the partially ferritic finished hot rolled strip can be subjected to a cooling interruption within a range of 0-20 seconds between the finishing rolling station and coiling. Also, in the event that the partially ferritic finished hot rolled strip is subjected to the cooling interruption, the strip is cooled at the cooling rate between 10-100 K/s before or after the cooling interruption and when the strip is at temperatures between 850-400° C.

In some instances, the coiled hot rolled strip has a tensile strength of at least 650 MPa, while in other instances the hot rolled strip has a tensile strength of at least 700 MPa. In addition, the tensile strength (TS) of the coiled hot rolled strip obeys the equation:

TS(MPa)=164.9+634.7×C %+53.6×Mn %+99.7×Si %+651.9×P %+3339.4×N %+11/√D_α,Accuracy±25 MPa  Eqn. 1

where D_α is the ferrite grain diameter (mm). In this manner, silicon alloyed steels with a desired tensile strength are produced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical illustration for a process according to an embodiment of the present invention;

FIG. 2A is an optical micrograph of a silicon alloyed steel etched with 2% nital at 100×;

FIG. 2B is an optical micrograph of the silicon alloyed steel shown in FIG. 2A at 500×;

FIG. 2C is an optical micrograph of the silicon alloyed steel shown in FIG. 2A at 1000×;

FIG. 3 is a graphical illustration of percent elongation versus tensile strength for two alloys according to an embodiment of the present invention and two comparison alloys; and

FIG. 4 is a graphical illustration of tensile strength versus coiling temperature for two alloys according to an embodiment of the present invention and a comparison alloy.

DETAILED DESCRIPTION OF THE INVENTION

A silicon alloyed steel for hot rolling and a process of hot rolling the silicon alloyed steel is provided. As such, the present invention has use as a structural material.

The silicon alloyed steel is suitable for hot rolling within a broad range of processing parameters. In addition, the steel alloy has a chemical composition, in wt %, within the range of 0.06-0.30 C, 0.3-2.0 Mn, 0.6-3.5 Si, and Fe plus other incidental melt impurities known to those skilled in the art. Optionally, the alloy has a phosphorus P content of less than or equal to 0.1, an Al content less than or equal to 0.08, a Cr content less than or equal to 0.6, a Mo content less than or equal to 0.6, a Ni content less than or equal to 0.3, a Cu content less than or equal to 0.6, a Nb content less than or equal to 0.08, a Ti content less than or equal to 0.10, and/or a S content less than or equal to 0.015. In some instances, the ratio of Si/Mn is between 1.3-3.5. In other instances, the ratio of Si/Mn is between 0.8-3.5 when the P content is between 0.03-0.05. Still in other instances, the ratio of Si/Mn is between 0.5-3.5 when the P content is between 0.05-0.1.

With such an alloy composition, the mechanical properties of the alloy are remarkably stable or uniform for a wide range of coiling temperatures. For example, the alloy can exhibit a spread of yield strength of less than 70 MPa, a spread of tensile strength less than 50 MPa, and a spread of percent elongation to failure less than 10% for coiling temperatures between 600-100° C. As such, a silicon alloyed steel suitable for hot rolling within a wide range of thermo-mechanical parameters is provided.

Turning now to FIG. 1, a graphical illustration of three preferred embodiments for hot forming a steel alloy with a chemical composition as described above is shown. The first embodiment includes first austenizing the alloy, followed by hot forming, followed by holding in air, followed by accelerated cooling and then coiling (path 1). The second embodiment includes first austenizing the alloy, followed by hot forming, followed by accelerated cooling, followed by slow cooling, e.g. cooling in air, and then coiling (path 2). The third embodiment includes first austenizing the alloy, followed by hot forming, followed by accelerated cooling directly to the coiling temperature (path 3). It is appreciated that the slope of the cooling curve for path 3 is for illustrative purposes only, i.e. the accelerated cooling rate for path 3 may or may not be the same as the accelerated cooling rate for path 1 and/or path 2. In some instances, the hot forming occurs in an austenitic finishing mode such that hot rolled strip produced according to the process has a microstructure with greater than 90 vol % ferrite plus pearlite before being coiled at a temperature equal to or less than 600° C. For the purposes of the present invention, the term “austenitic finishing mode” refers to finishing of the austenized material at a temperature above the Ar3 temperature for the steel. In addition, the Ar3 temperature is known to those skilled in the art to be the temperature at which point austenite begins to transform into ferrite during cooling. The holding in air is a cooling interruption that can occur between 0-20 seconds followed or preceded by cooling, e.g. on a run-out table at cooling rates between 10-100 K/s when the hot formed material is between temperatures of 950-400° C. It is appreciated that the hot forming can be hot rolling; however, this is not required. Stated differently, the hot forming can be a forging, extrusion or hot rolling operation.

In other instances, the austenized steel is subjected to a partially ferritic finishing mode which refers to finishing the material at a temperature below the Ar3 temperature for the alloy. Finishing the material in the partially ferritic mode still results in a microstructure with greater than 90 vol % ferrite plus pearlite before being coiled at a temperature that is less than or equal to 600° C. The material is also cooled between 10-100 K/s when the material is between 850° C. and the coiling temperature. Finally, and similar to the material hot formed in the austenitic finishing mode, the material hot formed in the partially ferritic finishing mode can be subjected to a cooling interruption of 0-20 seconds before or after accelerated cooling.

FIG. 2 provides three optical micrographs of a microstructure for a silicon alloyed steel according to an embodiment of the present invention. In particular, FIG. 2A is a micrograph of the microstructure etched with a 2% nital etching solution at 100×, whereas FIG. 2B is a micrograph at 500× and FIG. 2C is at 1000×. As shown in these optical micrographs, the microstructure consists of more than 90 vol % ferrite plus pearlite and generally has equiaxed grains with an ASTM grain size of approximately 11.0.

A steel alloy according to the embodiment of the present invention and processed as stated above has a yield strength of at least 400 MPa, a tensile strength of at least 600 MPa, and an elongation to failure of at least 20%. In addition, such properties are obtained when the hot rolled strip is coiled at any temperature or temperatures between 100-600° C. Stated differently, different coils of the alloy exhibit relatively constant mechanical properties despite coiling temperatures that can vary by as much as 400° C. As such, the steel alloy is relatively insensitive to processing parameters and exhibits mechanical properties that are relatively or essentially insensitive to coiling temperatures below 600° C. It is appreciated that such insensitivity to processing parameters naturally provides minimal head to tail variations in mechanical properties for coiled hot rolled strip and thus leads to exceptional yield for the material.

In order to provide specific examples of the steel alloy and yet not limit the scope of the invention in any way, two inventive alloy compositions and two comparison alloy compositions subjected to a range of finishing temperatures and coiling temperatures are discussed below.

A first inventive steel alloy (A) having a composition of 0.21 C, 1.01 Mn, 2.0 Si, and 0.06 Al was cast into a 160 by 160 mm thick ingot. The cast ingot was forged into a 20 mm specimen which was austenized at 1150° C. for 20 minutes and then subsequently subjected to four consecutive strokes of forging having a strain of 0.3 and a strain rate of 10/s in order to produce a final hot formed sample with a thickness of 6 mm.

A second inventive steel alloy (B) having a composition of 0.16 C, 1.00 Mn, 2.01 Si, and 0.06 Al was cast and forged in a similar manner as the first alloy composition.

The 6 mm specimens were cooled at accelerated cooling rates between 10-50 K/s between the finishing temperature and coiling temperature shown in the Table 1. In addition, a time period of approximately 10 seconds was present between the onset of accelerated cooling from the finishing temperature to the coiling temperature. Finally, the mechanical properties shown in Table 1 were produced from round tensile specimens having a diameter of 4 mm and a test length of 33.8 mm.

A first comparison alloy (C) having a composition of 0.201 C, 0.83 Mn, with the remainder being traces and unavoidable residuals was processed according to an embodiment illustrated in FIG. 1 with resultant mechanical properties and the range of temperatures for finishing and coiling shown in Table 2 below. In particular, alloy C was an Al-killed sheet material with a thickness of 4.7 mm. A second comparison alloy (D) having a composition of 0.23 C, 1.3 Mn, 0.15 Cr, with Ti and B microalloying was also processed according to an embodiment shown in FIG. 1 with resultant mechanical properties and the range of temperatures for finishing and coiling also shown in Table 2. In addition, alloy D was an Al-killed sheet material with a thickness between 1.8-2.2 mm.

TABLE 1
					TE (%)
Steel	FT (° C.)	CT (° C.)	YS (MPa)	TS (MPa)	(A5 DINEN)
Alloy A	860	350	551	714	31
	860	350	548	712	29
	800	400	550	701	25
	800	400	546	708	25.5
	830	400	586	725	26
	830	400	586	728	27
	860	400	573	732	29.5
	860	400	594	731	29
	890	400	573	721	31.5
	890	400	571	718	31
	860	450	539	719	28
	860	450	577	721	28.5
	860	350	554	725	27.5
	860	350	542	712	33
Alloy B	850	350	553	667	27
	850	350	535	661	31.5
	790	400	491	662	28.5
	790	400	509	663	30.5
	820	400	496	649	29.5
	820	400	486	646	30
	850	400	491	655	31.4
	850	400	518	660	32.9
	850	450	538	669	39
	850	450	530	681	32.5
	850	350	546	675	33.5
	850	350	526	669	32.5
	850	100	526	677	31
	850	100	512	686	29.5

As shown in Table 1, both alloy A and alloy B steel samples were subjected to a wide range of finishing temperatures and coiling temperatures. However, the mechanical properties exhibited by the tensile specimens were remarkably constant. For example, alloy A exhibited an average yield strength of 564 MPa, with a range or spread of 55 MPa, and average tensile strength of 719 MPa, with a range of 31 MPa, and an average tensile elongation of 29% with a range of 8% —even though samples were finished at temperatures between 800-890° C. and coiled at temperatures between 350-450° C.

Alloy B exhibited an average yield strength of 518 MPa, with a range of 67 MPa, an average tensile strength of 666 MPa, with a range of 40 MPa, and an average tensile elongation of 31% with a range of 12% —even though samples were finished at temperatures between 790-850° C. and coiled temperatures between 100-450° C.

TABLE 2
					TE (%)
Steel	FT (° C.)	CT (° C.)	YS (MPa)	TS (MPa)	(A5 DINEN)
Alloy C	873	665	366	482	30.7
	874	668	353	489	30.6
	887	658	298	462	36.7
	862	661	304	457	25.8
	866	663	307	445	38.0
	867	667	352	483	28.0
Alloy D	839	510	620	741	11.5
	847	503	636	772	11.4
	831	509	717	818	10.6
	840	514	544	697	15.8
	839	505	636	755	12.0
	832	511	728	820	9.8

In contrast, alloy C exhibited an average yield strength of 328 MPa, with a range of 68 MPa, and an average tensile strength of 470 MPa with a range of 44 MPa when coiled at temperatures that provided acceptable ductility, i.e. an average tensile elongation of 31.6% with a range of 12.2% —all for a range in coiling temperatures between 661-668° C. However, when alloy D was coiled at temperatures approaching 400° C., the average yield strength increased to 657 MPa, with a range of 184 MPa, the average tensile strength increased to 767 MPa, with a range of 123 MPa, ductility dropped to an average of 11.8%, with a range of 6% —all for a range of coiling temperatures between 503-514° C.

FIGS. 3 and 4 provide a graphical illustration of the variation, or lack thereof, of mechanical properties for the inventive alloys A and B when compared to the comparison alloys C and D. In particular, FIG. 3 illustrates how the elongation and tensile strength for the two inventive alloys A and B are relatively tightly grouped compared to the comparison steel D. In addition, the tensile strength of the two inventive alloys is significantly greater than the comparison steel C and also has comparable/equivalent ductility.

With regards to FIG. 4, the two inventive alloys A and B show a minimal range of tensile strength irrespective of being coiled at 100° C., 350° C., 400° C., or 450° C. In contrast, the comparison steel D when coiled at temperatures between 503-514° C. exhibited a relatively large spread in tensile strength when compared to the inventive alloys.

As illustrated by Table 1 and FIG. 4, inventive steel alloy A falls within a TS700 steel grade whereas inventive steel alloy B falls within a TS650 steel grade. Not being bound by theory, Equation 1 recited above was found to model the tensile strength of the inventive alloys and was thus used to develop additional inventive alloys having tensile strengths corresponding to the TS600 grade. Table 3 below provides compositions and calculated tensile strength for additional alloys that fall within the scope of the present invention.

	TABLE 3
						Ferrite grain	TS actual	TS model
	% C	% Mn	% Si	% P	% N	dia. (mm)	(MPa)	(MPa)
State of the art ferritic/
pearlitic structural steel
Comparison Alloy C	0.201	0.83	0.05	0.01	0.004	0.0079	470	486
Inventive Alloys
Alloy A “TS 700”	0.21	1.01	2	0.004	0.004	0.0067	719	702
Alloy B “TS 650”	0.16	1.00	2	0.004	0.004	0.007	666	667
Alloy E modeled for TS 600	0.13	0.8	1.6	0.01	0.006	0.007	n.a.	608
Alloy F modeled for TS 600	0.12	0.9	1.8	0.01	0.006	0.007	n.a.	627
Alloy G modeled for TS 650	0.18	0.7	1.4	0.01	0.006	0.007	n.a.	614
Alloy H modeled for TS 600	0.13	0.8	1.4	0.05	0.006	0.007	n.a.	614
Alloy I modeled for TS 600	0.12	0.9	1.6	0.05	0.006	0.007	n.a.	633
Alloy J modeled for TS 650	0.18	0.7	1.8	0.05	0.006	0.007	n.a.	680

The inventive alloys also exhibit acceptable or good weldability. Again, not being bound by theory, the inventive alloys do not exhibit excessive martensitic and hard phases in a heat affected zone (HAZ) due to their cooling transformation behavior. As such, and with the absence of martensite and hard phases, hydrogen induced cracking is reduced.

Given the above data and results, it is appreciated that a hot rolled high strength structural steel alloy that is insensitive to hot rolling finishing treatment temperatures and coiling temperatures is provided. It should also be appreciated that changes, modifications, and the like can be made to the alloy compositions and processing parameters and yet fall within the scope of the invention. As such, it is the claims and all equivalents thereof that define the scope of the invention.

Claims

1. A process for making a hot rolled strip from a silicon alloyed steel, the process comprising:

providing a steel slab having a chemical composition in weight percent within a range of 0.06-0.30 C, 0.3-2.0 Mn, 0.6-3.5 Si and Fe plus incidental melting impurities;

hot rolling the steel slab to produce a hot rolled steel strip;

and coiling the hot rolled strip at coiling temperatures between 100-600° C., the coiled hot rolled strip having a microstructure containing at least 90 vol % ferrite plus pearlite, a yield strength of at least 400 MPa, a tensile strength of at least 600 MPa and a tensile elongation of at least 20% irrespective of the hot rolled strip being coiled at 100° C., 350° C. or 500° C.

2. The process of claim 1, wherein the steel alloy has 0.1 max P, 0.08 max Al, 0.6 max Cr, 0.3 max Ni, 0.6 max Cu, 0.08 max Nb, 0.10 max Ti and 0.012 max S.

3. The process of claim 2, wherein the steel alloy has a Si/Mn ratio between 1.3-3.5.

4. The process of claim 2, wherein the steel alloy has between 0.03-0.05 P and a Si/Mn ratio between 0.8-3.5.

5. The process of claim 2, wherein the steel alloy has between 0.05-0.1 P and a Si/Mn ratio between 0.3-3.5.

6. The process of claim 1, wherein the steel alloy is hot rolled in an austenitic finishing roll mode and cooled at a cooling rate between 10-100 K/s when the hot rolled strip is between 950-400° C.

7. The process of claim 6, further including a cooling interruption within a range of 3-20 seconds of the hot rolled strip between austenitic finishing rolling and coiling.

8. The process of claim 6, wherein the hot rolled strip is cooled directly from the austentic finishing roll mode to the coiling temperature at the cooling rate between 10-100 K/s.

9. The process of claim 1, wherein the steel alloy is hot rolled in a partially ferritic finishing roll mode and cooled at a cooling rate between 10-100 K/s when the hot rolled strip is between 850-400° C.

10. The process of claim 9, further including a cooling interruption within a range of 3-20 seconds of the hot rolled strip between partially ferritic finishing rolling and coiling.

11. The process of claim 9, wherein the hot rolled strip is cooled directly from the partially ferritic finishing roll mode to the coiling temperature at the cooling rate between 10-100 K/s.

12. The process of claim 1, wherein the coiled hot rolled strip has a tensile strength of at least 650 MPa.

13. The process of claim 1, wherein the tensile strength is at least 700 MPa.

14. A process for making a high strength steel sheet comprising: irrespective of the hot rolled strip being coiled at temperatures between 100-600° C.

providing a steel slab with a thickness between 50 and 280 mm, the steel slab having a chemical composition in weight percent within a range of 0.06-0.30 C, 0.3-2.0 Mn, 0.6-3.5 Si, 0.1 max P, 0.08 max Al, 0.6 max Cr, 0.3 max Ni, 0.6 max Cu, 0.08 max Nb, 0.10 max Ti, 0.012 max S, balance Fe and incidental melting impurities;

soaking the steel slab at temperatures between 1150 and 1350° C.;

hot rolling the steel alloy to produce a hot rolled steel strip;

and coiling the hot rolled strip at coiling temperatures between 100-600° C., the coiled hot rolled strip having a microstructure containing at least 90 vol % ferrite plus pearlite, a yield strength of at least 400 MPa, a tensile elongation of at least 20% and a tensile strength that obeys the relation: TS(+/−25 MPa)=164.9+634.7×C %+53.6×Mn %+99.7×Si %+651.9×P %+3339.4×N %+11/√Dα

15. The process of claim 14, wherein the steel alloy is hot rolled in an austenitic finishing roll mode and cooled at a cooling rate between 10-100 K/s when the hot rolled strip is between 950-400° C.

16. The process of claim 15, further including a cooling interruption within a range of 3-20 seconds of the hot rolled strip between austenitic finishing rolling and coiling.

17. The process of claim 16, wherein the hot rolled strip is cooled directly from the austenitic finishing roll mode to the at the cooling rate between 10-100 K/s.

18. The process of claim 14 wherein the steel alloy is hot rolled in a partially ferritic finishing roll mode and cooled at a cooling rate between 10-100 K/s when the hot rolled strip is between 850-400° C.

19. The process of claim 18, further including a cooling interruption within a range of 3-20 seconds of the hot rolled strip between partially ferritic finishing rolling and coiling.

20. The process of claim 19, wherein the hot rolled strip is cooled directly from the partially ferritic finishing roll mode to the coiling temperature at the cooling rate between 10-100 K/s.