Hot rolled steel plate with high wear resistance and method of manufacturing the same

Abstract

A hot rolled steel plate having a composition including, by weight percent: C: 0.10-0.25%, Mn: 3.0-5.0%, Si: 0.80-1.60%, Al: 0.10%-0.60%, S≤0.010%, P≤ 0.020%, N≤0.008% the remainder of the composition being iron and unavoidable impurities resulting from the smelting, and having a microstructure consisting of, in surface fraction: from 5 to 10% of residual austenite, the rest being auto-tempered martensite.

Description

The present invention relates to a hot rolled steel plate having high wear resistance, high strength and high toughness, and to a method to obtain such steel plate.

BACKGROUND

In the mineral industry, water quenched martensitic steels are used today to manufacture wear components, for applications in mines and quarries, cement and steelmaking industries, public works and agricultural machinery. In those fields, wear damage produced by heavy impacts and scratches is a major issue requiring repetitive machine downtown and maintenance costs to change worn components such as liners, buckets and jaw crushers. Considerable effort is made to prolong the lifetime of wear parts and a large variety of materials and designs is commonplace. To obtain a significant increase in wear resistance, according to the applications, hardness should be higher than 440 HB. Knowing the impact toughness is closely related to the wear resistance, this property must be the highest, but a classical water quench martensitic steel presents a low impact toughness.

Moreover, in view of the global environmental conservation, the steel making industry is looking for a less-energy-consuming fabrication route.

SUMMARY OF THE INVENTION

It is an object of the present invention to solve the above-mentioned problem and to provide a steel plate having high hardness above or equal to 400HB and preferably to 440HB at mid thickness of the plate and above or equal to 430HB and preferably to 450 HB at the surface of the plate, and high toughness with Charpy impact energy at −40° C. above or equal to 25 J, and easily processable on conventional process route.

Preferably the hot rolled steel plate has a yield strength YS above or equal to 970 MPa and preferably above or equal to 980 MPa.

Preferably the hot rolled steel plate has a tensile strength TS above or equal to 1400 MPa.

The present invention provides a hot-rolled steel plate, made of a steel having a composition comprising, by weight percent:

• • C: 0.10-0.25%
  • Mn: 3.0-5.0%
  • Si: 0.80-1.60%
  • Al: 0.10%-0.60%
  • S≤0.010%
  • P≤0.020%
  • N≤0.008%

and comprising optionally one or more of the following elements, in weight percentage:

• • B: 0.0003-0.004%
  • Ti≤0.06%
  • Nb≤0.05%
  • Mo≤0.3%
  • Cr≤0.80%
  • Cu≤0.2%
  • Ni≤0.30%

the remainder of the composition being iron and unavoidable impurities resulting from the smelting, said steel plate having a microstructure comprising, in surface fraction,

• • from 5 to 10% of residual austenite,
  • the rest being auto-tempered martensite.

The present invention also provides a method for manufacturing a hot-rolled steel plate, comprising the following successive steps:

• • casting a steel to obtain a semi-product, said semi product having a composition as described above,
  • reheating the semi-product to a temperature T_reheat of 1100° C. to 1300° C.,
  • hot rolling the semi-product with a finish hot rolling temperature FRT above Ac1,
  • optionally heating the steel plate to a temperature T_H of 850° C. to 950° C., and maintained at said temperature for a holding time below or equal to 30 minutes.
  • optionally quenching the steel plate from T_H to a temperature T_Q below 400° C., in order to obtain at mid thickness of the steel plate a temperature of 300° C. to 400° C. at the end of the quenching,
  • and air cooling the steel plate to room temperature.

DETAILED DESCRIPTION

The present invention will now be described in detail and illustrated by examples without introducing limitations.

The composition of the steel according to the invention will now be described, the content being expressed in weight percent.

The carbon content is from 0.10% to 0.25%. If the carbon content is too high, the weldability of the steel is insufficient. If the carbon content is lower than 0.10%, the austenite fraction is not stabilized enough to obtain targeted properties. In a preferred embodiment of the invention, the carbon content is from 0.15% and 0.20%.

The manganese content is from 3.0% to 5.0%. Above 5.0% of addition, the risk of central segregation increases to the detriment of the toughness. Below 3.0%, the final structure comprises an insufficient retained austenite fraction to obtain the desired properties. In a preferred embodiment of the invention the manganese content is between 3.5% and 4.5%.

According to the invention, the silicon content is from 0.80% to 1.60%. A silicon addition of at least 0.80% helps to stabilize a sufficient amount of retained austenite. Above 1.60%, silicon is detrimental for toughness. Moreover, silicon oxides form at the surface, which impairs the coatability of the steel. In a preferred embodiment of the invention, the silicon content is from 1.00% to 1.60%.

The aluminium content is from 0.10% to 0.60%, as it is a very effective element for deoxidizing the steel in the liquid phase during elaboration. Moreover, aluminum improves weldability of the steel. The aluminum content is lower than 0.60% to avoid the occurrence of inclusions and to avoid oxidation problems.

Optionally some elements can be added to the composition of the steel according to the invention.

The boron content can be from 0.0003% and 0.004%. The presence of boron can increase the toughness. Moreover, boron improves weldability of the steel. Above 0.004%, the formation of borocarbides at the prior austenite grain boundaries is promoted, making the steel more brittle. Below 0.0003%, there is not a sufficient concentration of free B that segregates at the prior austenite grain boundaries to increase toughness of the steel.

Titanium can be added optionally up to 0.06% to provide precipitation strengthening. Preferably a minimum of 0.01% of titanium is added in addition of boron to protect boron against the formation of BN.

Niobium can be added up to 0.05% to refine the austenite grains during hot-rolling and to provide precipitation strengthening. Preferably, the minimum amount of niobium added is 0.0010%.

Molybdenum can optionally be added, at a maximum of 0.3%. Molybdenum stabilizes the austenite and increases toughness of the steel. Moreover, molybdenum improves weldability of the steel. Above 0.3%, the addition of molybdenum is costly and ineffective in view of the properties which are required.

Preferably, the minimum amount of molybdenum is 0.0010%.

A maximum of 0.80% of chromium is allowed. Above, a saturation effect is noted, and adding chromium is both useless and expensive. Preferably, the minimum amount of chromium is 0.0010%.

Copper can be added up to 0.2% in order to increase the toughness of the steel.

Nickel can be added up to 0.30% to limit the risk of delayed fracture due to hydrogen embrittlement.

The remainder of the composition of the steel is iron and impurities resulting from the smelting. In this respect, P, S and N at least are considered as residual elements which are unavoidable impurities. Their content is below or equal to 0.010% for S, below or equal to 0.020% for P and below or equal to 0.008% for N.

The microstructure of the hot rolled steel plate according to the invention will now be described.

The hot rolled steel plate has a microstructure consisting of, in surface fraction, from 5% to 10% of retained austenite, the balance being auto tempered martensite.

The steel plate is hot rolled with a final rolling temperature FRT above Ac1.

In a preferred embodiment of the invention, the FRT is below Ac3 and the microstructure consists of ferrite and austenite. During the subsequent cooling, ferrite and a part of austenite are transformed in martensite.

In another preferred embodiment of the invention, the FRT is above or equal to Ac3, and the microstructure is fully austenitic. During the subsequent cooling, a part of austenite is transformed in martensite.

In a preferred embodiment of the invention, in which the steel plate has a thickness above or equal to 5 mm and below 40 mm, the martensite is auto tempered, because of the slow cooling rate of the subsequent cooling, which leads to obtain the targeted mechanical properties. Preferably, the steel plate can have a thickness above 25 mm, and below 40 mm.

In another preferred embodiment of the invention, in which the steel plate has a thickness above or equal to 40 mm and below or equal to 100 mm, a heating step up to a temperature T_H from 850° C. to 950° C. is necessary to obtain a homogeneous microstructure of austenite in the thickness of the plate. During the quenching step down to a temperature T_Q below 400° C., a part of this austenite is transformed in martensite. This martensite is then auto-tempered during the subsequent air cooling.

A part of the austenite remains in the final microstructure. This retained austenite contributes to obtain a great wear resistance and is beneficial for toughness. During wear damage and impacts, the retained austenite will be transformed to martensite inducing a volume expansion from the fcc to bcc structure, which can fill impact cracks and stop their propagation. The TRIP effect induces also a great work-hardening in service, which largely contributes to improve the wear resistance of the bare material.

The steel plate according to the invention can be produced by any appropriate manufacturing method and the person ordinary skilled in the art can define one. It is however preferred to use the method according to the invention comprising the following steps:

A semi-product able to be further hot-rolled, is provided with the steel composition described above. The semi product is heated to a temperature of 1100° C. to 1300° C., to make it possible to ease hot rolling, with a final hot rolling temperature FRT above Ac1. Preferably, the FRT is above Ac1+100° C., more preferably above Ac1+150° C. When FRT is lower than Ac1, the microstructure will not contain enough austenite to ensure the hardness and toughness of the plate.

In a preferred embodiment of the invention, the FRT is below Ac3.

In another preferred embodiment of the invention, the FRT is above or equal to Ac3.

The hot-rolled steel plate having a thickness above or equal to 5 mm and below 40 mm is then air cooled. Preferably, the cooling rate at mid thickness of the plate is below 5° C./s.

From 40 mm of thickness, and up to 100 mm of thickness, the steel plate is reheated to a temperature T_H of 850° C. to 950° C. and maintained at said temperature for a holding time below or equal to 30 minutes, in order to obtain an homogeneous microstructure in all the thickness of the plate. The plate is then quenched at a cooling rate higher than 1° C./s to a temperature T_Q below 400° C., in order to obtain at mid thickness of the steel plate a temperature of 300° C. to 400° C. at the end of the quenching, before being air cooled to room temperature.

Calculations done with the use of a software like Forge® allow to determine the duration of the quenching to obtain a temperature of 300° C. to 400° C. at mid thickness of the steel plate at the end of the quenching. Preferably the quenching temperature T_Q is above or equal to 200° C., more preferably above or equal to 250° C.

The invention will be now illustrated by the following examples, which are by no way limitative.

Example

Four grades, whose compositions are gathered in table 1, were cast in semi-products and processed into steel plates.

Table 1—Compositions

The tested compositions are gathered in the following table wherein the element contents are expressed in weight percent.

															Ac1	Ac3
Steel	C	Mn	Si	Al	S	P	N	B	Ti	Nb	Mo	Cr	Cu	Ni	(° C.)	(° C.)
A	0.20	3.8	1.30	0.33	0.0004	0.011	0.003	0.0014	0.044	0.024	0.16	0.21	0.13	0.12	415	880
B	0.20	1.5	0.35	0.02	0.0026	0.010	0.007	0.0004	0.044	0.002	0.18	1.65	0.20	0.19	750	829
C	0.26	1.2	0.31	0.05	0.0006	0.008	0.005	0.0031	0.037	0.003	0.13	0.76	0.17	0.13	738	817
D	0.18	1.4	0.80	0.02	0.014	0.009	0.005	0	0.02	0.005	0.16	1.1	0.33	0.4	750	850
Underline values: not corresponding to the invention

Steel A is according to the invention, B-D are not according to the invention.

Ac1 and Ac3 of steel A have been determined through dilatometry measures.

Ac1 and Ac3 of steels B-D have been determined through a formula, based on the weight percent of the corresponding elements:

$\mathrm{Ac}3=925-219\surd \left(\%C\right)-7\%Mn+39\%Si-16\%Ni+13\%Mo,$ $\mathrm{Ac}1=742-29\%C-14\%Mn+13\%Si-16\%Cr-17\%Ni+16\%Mo+36\%Cu.$

Table 2—Process Parameters

Steel ingots were heated at 1250° C., and hot rolled with a final rolling temperature FRT to obtain a hot rolled steel plate with a thickness d. The hot rolled steel plate of trials 1-3, 6 and 8 are then air cooled. For trials 4 and 5, the steel plates are then heated to a temperature T_H and quenched to room temperature (RT) at various cooling rate. In trial 7, the steel plate having a thickness of 40 mm is heated up to a temperature T_H and quenched to a temperature T_Q higher than RT, in order to obtain at mid thickness of the steel plate a temperature of 300° C. to 400° C. at the end of the quenching, before being air cooled. The following specific conditions were applied:

	Hot rolling	Cooling		Quenching
		FRT		rate	TH	Cooling	TQ
Trial	Steel	(° C.)	d(mm)	(° C./s)	(° C.)	rate (° C./s)	(° C.)
1	A	670	5	1.44	—	—	—
2	A	850	10	0.75	—	—	—
3	A	970	20	0.40	—	—	—
4	B	1030	30	0.28	900	0.98	RT
5	C	1070	30	0.28	900	16.7	RT
6	D	850	10	0.75	—	—	—
7	A	1055	40	0.21	920	17	350
8	A	1037	30	0.28	—	—	—
Underline values: not corresponding to the invention

The hot rolled plates were then analyzed and the corresponding microstructure elements and mechanical properties were respectively gathered in tables 3 and 4.

Table 3—Microstructure of the Hot Rolled Steel Plate

The phase percentages of the microstructure of the obtained hot rolled steel plate were determined at quarter thickness d/4 of the steel plate.

	Residual	Auto-tempered
Trial	Austenite (%)	Martensite (%)	Martensite(%)	Bainite (%)
1	7	93	—	0
2	9	91	—	0
3	7	93	—	0
4	7	68	—	25
5	0	0	100	0
6	0	10	—	90
7	5	95	—	0
8	6	94	—	0
Underlined values: not corresponding to the invention

The surface fractions of phases in the microstructure are determined through the following method: a specimen is cut from the hot rolled plate, polished and etched with a reagent known per se, to reveal the microstructure. The section is afterwards examined through scanning electron microscope, for example with a Scanning Electron Microscope with a Field Emission Gun (“FEG-SEM”) at a magnification greater than 5000×, in secondary electron mode.

The determination of the surface fraction of retained austenite and auto tempered martensite is performed thanks to SEM observations after Nital or Picral/Nital reagent etching and thanks to X-ray diffraction analysis.

Table 4—Mechanical Properties of the Hot Rolled Steel Plate

Mechanical properties of the tested samples were determined and gathered in the following table. The hardness has been determined at the surface of the steel plate (down to 2 mm below the top surface) and at mid thickness (d/2) of the steel plate. Hardness is measured according to Standard ASTM E10. Charpy impact energy at −40° C. is measured according to Standard ISO 148-1:2006 (F) and ISO 148-1: 2017 (F). YS and TS are measured according to ISO standard ISO 6892-1.

				Charpy impact
	Hardness (HB)	YS	TS	energy at −40° C.
Trials	Top surface	d/2	(MPa)	(MPa)	(J)
1	470	460	1380	1500	45
2	460	455	1030	1550	42
3	480	460	990	1470	39
4	380	360	959	1363	52
5	450	435	1257	1521	10
6	330	315	830	1130	12
7	441	416	988	1509	33
8	437	421	1015	1459	38
Underlined values: do not match the targeted values

The examples show that the steel plates according to the invention, namely trials 1 to 3 and 7-8 are the only ones to show the targeted property, thanks to their specific compositions and microstructures.

The steel part of trials 4-6 have a chemical composition with lower manganese. The quenching step in trial 4 is performed down to room temperature at a slow cooling rate. A part of austenite is then transformed in martensite, being auto tempered because of the slow cooling but also in bainite, which is detrimental for hardness. In trial 5, the quenching step is done down to room temperature, at a higher cooling rate. Austenite is thus totally transformed into martensite. The absence of austenite is detrimental for toughness.

Moreover, Trials 2 and 6 undergo the same process, the difference being in the chemical composition. The low level of manganese combined to the absence of boron in trial 6 lead to low toughness.

The high thickness steel plate of trial 7 is reheated and quenched. This quenching step is interrupted at a temperature of 350° C. before being air cooled to room temperature. Austenite formed during the heating step is then partly transformed into martensite. This martensite is auto-tempered during the air cooling step, leading to the high hardness and toughness level.

Claims

1-2. (canceled)

3. A hot-rolled steel plate, made of a steel having a composition comprising, by weight percent: and optionally one or more of the following elements, in weight percentage: a remainder of the composition being iron and unavoidable impurities resulting from processing,

C: 0.10-0.25%

Mn: 3.0-5.0%

Si: 0.80-1.60%

Al: 0.10%-0.60%

S≤0.010%

P≤0.020%

N≤0.008%

B: 0.0003-0.004%

Ti≤0.06%

Nb≤0.05%

Mo≤0.3%

Cr≤0.80%

Cu≤0.2%

Ni≤0.30%

the steel plate having a microstructure comprising, in surface fraction, from 5 to 10% of residual austenite, a rest being auto-tempered martensite.

4. A method for manufacturing a hot-rolled steel plate, comprising the following successive steps:

casting a steel to obtain a semi-product, the semi product having a composition comprising, by weight percent: C: 0.10-0.25% Mn: 3.0-5.0% Si: 0.80-1.60% Al: 0.10%-0.60% S≤0.010% P≤0.020% N≤0.008%

and optionally one or more of the following elements, in weight percentage: B: 0.0003-0.004% Ti≤0.06% Nb≤0.05% Mo≤0.3% Cr≤0.80% Cu≤0.2% Ni≤0.30%

a remainder of the composition being iron and unavoidable impurities resulting from processing,

reheating the semi-product to a temperature Treheat of 1100° C. to 1300° C.;

hot rolling the semi-product with a finish hot rolling temperature FRT above Ac1 to obtain a steel plate; and

air cooling the steel plate to room temperature.

5. The method as recited in claim 4 further comprising heating the steel plate to a temperature TH of 850° C. to 950° C., and maintaining at the temperature for a holding time below or equal to 30 minutes.

6. The method as recited in claim 5 further comprising quenching the steel plate from TH to a temperature TQ below 400° C., in order to obtain at mid thickness of the steel plate a temperature of 300° C. to 400° C. at the end of the quenching.

7. The method as recited in claim 4 further comprising quenching the steel plate from TH to a temperature TQ below 400° C., in order to obtain at mid thickness of the steel plate a temperature of 300° C. to 400° C. at the end of the quenching.